SMPS(flyback question)

[[email protected]]

Hi to all.
I have a quick question about a flyback convertor I'm working on.
Its 220V ac in 13.7V dc out. I'm using a uc3842 contriller chip.
At 60W ( about 4.5A) load everything seems good. At about 80 to 100W
the cct starts to
get unstable. I can hear a hissing sound etc. Looking at the switching
FET's
drain waveform I can see that the frequency is changing ( hopping
around). I have found that if
I keep the output diode(BYV32) cool (with a fan) then the stability
problems
go away. The diode is mounted on a small heatsink. At 50W the heatsink
is just to
hot to touch for more than say 5-10s( probable about 60 deg c , hav'nt
measured). Obviously at higher currents it gets a lot hotter.Why would
the diode temperature cause the instability at higher temperatures?
The obvious solution is to use a bigger heatsink , which I will be
doing , but I still would like to know whats happening at higher temps
to cause instability.
I am far from an expert with SMPS's , so any pointers/help will be
appreciated.
Hope I have not been to vague.
Cheers
Rob

 

[budgie]

Hi to all.
I have a quick question about a flyback convertor I'm working on.
Its 220V ac in 13.7V dc out. I'm using a uc3842 contriller chip.
At 60W ( about 4.5A) load everything seems good. At about 80 to 100W
the cct starts to
get unstable. I can hear a hissing sound etc. Looking at the switching
FET's
drain waveform I can see that the frequency is changing ( hopping
around). I have found that if
I keep the output diode(BYV32) cool (with a fan) then the stability
problems
go away. The diode is mounted on a small heatsink. At 50W the heatsink
is just to
hot to touch for more than say 5-10s( probable about 60 deg c , hav'nt
measured). Obviously at higher currents it gets a lot hotter.Why would
the diode temperature cause the instability at higher temperatures?
The obvious solution is to use a bigger heatsink , which I will be
doing , but I still would like to know whats happening at higher temps
to cause instability.
I am far from an expert with SMPS's , so any pointers/help will be
appreciated.

I would normally use a Schottky diode every time rather than waste time on a
so-called ultra-fast type. I have inherited SMPS units with issues, and
surprisingly often a change to Schottky diode(s) changes the whole unit back to
proper operation, and with a dramatic reduction in diode temperature.

 

[[email protected]]

Thanks , I'll have a look around for a suitable replacement.
Cheers
Rob

 

[John_H]

Hi to all.
I have a quick question about a flyback convertor I'm working on.
Its 220V ac in 13.7V dc out. I'm using a uc3842 contriller chip.
At 60W ( about 4.5A) load everything seems good. At about 80 to 100W
the cct starts to
get unstable. I can hear a hissing sound etc. Looking at the switching
FET's
drain waveform I can see that the frequency is changing ( hopping
around). I have found that if
I keep the output diode(BYV32) cool (with a fan) then the stability
problems
go away. The diode is mounted on a small heatsink. At 50W the heatsink
is just to
hot to touch for more than say 5-10s( probable about 60 deg c , hav'nt
measured). Obviously at higher currents it gets a lot hotter.Why would
the diode temperature cause the instability at higher temperatures?
The obvious solution is to use a bigger heatsink , which I will be
doing , but I still would like to know whats happening at higher temps
to cause instability.
I am far from an expert with SMPS's , so any pointers/help will be
appreciated.
Hope I have not been to vague.
Cheers
Rob

The hissing could be a side effect. When I was working with a
uc3842-based flyback supply in 1988, I heard some instability that most
people didn't that I quickly traced back to feedback circuit that drove
the optoisolator. Check to make sure that your feedback is stable at
those higher loads.

 

[Paul Mathews]

1) It's not necessarily a bad thing for a rectifier to run hot, since
efficiency increases with the consequent decrease in forward voltage.
2) At higher output power, peak current obviously rises, and this has
many effects that lead to instability, especially: increased energy
stored in leakage inductance --> larger voltage spikes --> instability
due to noise
3) Duty factor also increases: research on keywords 'sub-harmonic
oscillation' and 'ramp compensation'

Changing the type of rectifier diode will sometimes help out with #2
above.

Paul Mathews

 

[John Popelish]

Why would
the diode temperature cause the instability at higher temperatures?
The obvious solution is to use a bigger heatsink , which I will be
doing , but I still would like to know whats happening at higher temps
to cause instability.

Just a guess... At higher current, the supply is more likely to hit
continuous current mode (inductor does not fully discharge before end
of cycle). This means that the power switch is turning on, while the
output diode is still conducting. The reverse recovery diode current
gets added to the normal switch current (normally, mostly capacitive
current from winding capacitance) during turn on, and this normally
gets higher when the diode gets hotter. At some point, the switch may
be hitting a current limit and skipping a cycle (causing the audible
noise).

 

[[email protected]]

Just a guess... At higher current, the supply is more likely to hit
continuous current mode (inductor does not fully discharge before end
of cycle). This means that the power switch is turning on, while the
output diode is still conducting. The reverse recovery diode current
gets added to the normal switch current (normally, mostly capacitive
current from winding capacitance) during turn on, and this normally
gets higher when the diode gets hotter. At some point, the switch may
be hitting a current limit and skipping a cycle (causing the audible
noise).

Hi to all.
Thanks for the pointers. Would I be able to see if the supply is
running in continuous
mode by monitoring the voltage across the current sense resistor. I
assume I will , but
I am not sure what to look for exactly. Fiddeling around in the past I
have noticed 2 types of voltage across the sense resistor. One that
rises from 0 in a linear ramp until turn off of the transistor , and
one that rises sharply to some value , and then ramps linearly until
transistor turn off. Is this examples of the circuit running in
discontinous and continous modes.
As I mentioned I am still feeling my way in this stuff.
Thanks for the help.
Cheers
Rob

 

[John_H]

Hi to all.
Thanks for the pointers. Would I be able to see if the supply is
running in continuous
mode by monitoring the voltage across the current sense resistor. I
assume I will , but
I am not sure what to look for exactly. Fiddeling around in the past I
have noticed 2 types of voltage across the sense resistor. One that
rises from 0 in a linear ramp until turn off of the transistor , and
one that rises sharply to some value , and then ramps linearly until
transistor turn off. Is this examples of the circuit running in
discontinous and continous modes.
As I mentioned I am still feeling my way in this stuff.
Thanks for the help.
Cheers
Rob

To see if you're continuous or not, I'd suggest looking at the
secondary-side (output) diode. When the diode is negatively biased, the
transformer is taking in energy on the primary side. When the diode is
forward biased, the transformer is dumping its stored energy into the output
caps. When the diode drops from its Vf (and may oscillate slightly negative
to Vf) but doesn't kick back over to extremely reverse biased, the
transformer is "empty" and you're in discontinuous mode.

 

[Fred Bartoli]

Just a guess... At higher current, the supply is more likely to hit
continuous current mode (inductor does not fully discharge before end
of cycle). This means that the power switch is turning on, while the
output diode is still conducting. The reverse recovery diode current
gets added to the normal switch current (normally, mostly capacitive
current from winding capacitance) during turn on, and this normally
gets higher when the diode gets hotter. At some point, the switch may
be hitting a current limit and skipping a cycle (causing the audible
noise).

Good guess, I guess. With the additional effect that a flyback going
continuous conduction gives you an annoying RHP zero that won't do any good
to the feedback loop stability if not accounted for.

To Rob,
check your primary side switch current and see whether the current ramp
starts at zero (discontinuous mode) or if the current jumps to a non null
value on MOS switch on (continuous mode).
Alternatively you can look at the voltages waveforms. In CC mode you have
only 2 voltages levels, corresponding to the ON/OFF states of the switch. In
DC mode you'll see 3 levels, the 2 of CC mode plus one idle voltage when all
the core stored energy has been transfered to the load side. During this
phase you'll see some damped voltage ringing (the transformer magnetizing
inductance is resonating with all the parasitics capacitance it sees).

If John guessed right, you should have a zero pulse start below your
observed limit, and a rising non zero pulse start above that same limit.

CC is often not desired because of the higher switching losses, and because
the RHPZ gives you a slower transient response.
In that case the cure could be to redesign the transformer.

OTOH DC puts more stress on semiconductors (peak currents are higher).

 

[Paul Mathews]

Yes. Sense resistor waveform will be trapezoid (plus a leading edge
spike) for continuous mode versus triangle (plus spike) for
discontinuous. Subharmonic oscillation becomes more likely as you
increase duty factor to near 50% and beyond. However, you could easily
have multiple interacting causes of instability if your layout and
construction have excessive parasitics.
Paul Mathews

 

[John Popelish]

Thanks for the pointers. Would I be able to see if the supply is
running in continuous
mode by monitoring the voltage across the current sense resistor. I
assume I will , but
I am not sure what to look for exactly.

The easiest place to see it is by looking at the secondary voltage
feeding the rectifier. If the current is discontinuous, there will be
a small period of ringing before each charge up pulse. When the
current becomes continuous, the voltage will jerk from forward biasing
the rectifier right to the inverse charge up voltage.

Fiddeling around in the past I
have noticed 2 types of voltage across the sense resistor. One that
rises from 0 in a linear ramp until turn off of the transistor , and
one that rises sharply to some value , and then ramps linearly until
transistor turn off. Is this examples of the circuit running in
discontinous and continous modes.

Yes. Continuous current mode means the current never ramps all the
way to zero between charge ups. Firing power into a conducting
rectifier is more likely to produce a brief spike on the current sense
resistor that trips the current limit, even though the current after
the spike is well below the limit. Sometimes adding a small RC time
constant (shorter than the normal ramp time) between the current sense
resistor and the current sense circuit, will smear this spike well
enough to let the supply run in continuous current mode. However,
both the rectifier and power switch will be running hotter because of
the spike.

 

[[email protected]]

The easiest place to see it is by looking at the secondary voltage
feeding the rectifier. If the current is discontinuous, there will be
a small period of ringing before each charge up pulse. When the
current becomes continuous, the voltage will jerk from forward biasing
the rectifier right to the inverse charge up voltage.


Yes. Continuous current mode means the current never ramps all the
way to zero between charge ups. Firing power into a conducting
rectifier is more likely to produce a brief spike on the current sense
resistor that trips the current limit, even though the current after
the spike is well below the limit. Sometimes adding a small RC time
constant (shorter than the normal ramp time) between the current sense
resistor and the current sense circuit, will smear this spike well
enough to let the supply run in continuous current mode. However,
both the rectifier and power switch will be running hotter because of
the spike.

Hi all.
Thanks to all who have replied to my questions.
The voltage I am measuring across the sense resistor is trapazoidal at
all but very low power levels.Does this mean I'm running in continous
mode most of the time?
I'm not sure I understand the posts about taking the voltage from the
input of the diode.
At the input of the diode I see a (almost perfect) square wave , 25%
duty cycle going from -50 to 14V. There is a little ringing at the top
of the rising edge and the bottom of the falling edge , but that is
all.
The primary inductance is about 600uh , sec inductance around 18uH and
the switching frequency is about 110Khz.The current sense resistor is
0.375 ohms.
The feedback to the current sence input to the 3842 is via a 1k with
470pF to gnd.
Thanks to all who are helping me here.
Cheers
Rob

 

[Terry Given]

Hi all.
Thanks to all who have replied to my questions.
The voltage I am measuring across the sense resistor is trapazoidal at
all but very low power levels.Does this mean I'm running in continous
mode most of the time?
yep

I'm not sure I understand the posts about taking the voltage from the
input of the diode.
At the input of the diode I see a (almost perfect) square wave , 25%
duty cycle going from -50 to 14V. There is a little ringing at the top
of the rising edge and the bottom of the falling edge , but that is
all.

thats cos its ccm

The primary inductance is about 600uh , sec inductance around 18uH and
the switching frequency is about 110Khz.The current sense resistor is
0.375 ohms.
The feedback to the current sence input to the 3842 is via a 1k with
470pF to gnd.
Thanks to all who are helping me here.
Cheers
Rob

whats the load & Vout?

cheers
terry

 

[[email protected]]

Hi all again.
Doing a quick simulation shows it definately seems to be running in CC
mode. There is still significant current flowing
in the secondary winding then the next cycle begins.
What ways are ther to solve this.
The output voltage is 13.7V
Load is 4.6A ( 12V halogen bulb)

From the simulation I see that reducing the output caps improves the

situation. Still CC mode but the current has ramped much closer to 0
before switching begins again. I currently have 1000uF -> 10uH - >
1000uF output config. Reducing the caps to 220u improves matters.I'll
do some more experimenting here.
I could reduce the swithcing frequency I suppose , give more time for
the current to decay to 0 before the next cycle , but I'd rather not.
If I have to redo the transformer , what do I change?

 

[Tim Williams]

If I have to redo the transformer , what do I change?

More turns on the primary (or less on the secondary), I would say.

Tim

 

[[email protected]]

Hi there.
Reading a bit it seems that CC mode is not a bad mode to be running in.
It may be better to change
my compensation to account for this.The datasheet for the uc3842 shows
compensation for
for a flyback in cc mode.It does not show values unfortunately. Not
realy sure how to calculate them.
It is basically just another resistor and cap to create another pole
,but at what frequency.
This is where my ignorance on the subject becomes sorely apparent!!
Bit more experimenting required.
The circuit is basically running quite well. I can deliver up to 150W(
have not pushed it any further)
with a fan heeping things cool.The switching tranny and diode are only
on small heatsinks (20 *30 mm with 2 forward facing
sections of about 10*30mm)
As I said , bit more experimenting required :0>
Cheers
Rob

 

[John Popelish]

Hi there.
Reading a bit it seems that CC mode is not a bad mode to be running in.

I agree. It lowers the RMS winding current relative to the DC output
current and in some ways, lowers the noise the supply emits. It also
has some negatives, like slower control response to output error and
rougher turn on transient.

It may be better to change
my compensation to account for this.The datasheet for the uc3842 shows
compensation for
for a flyback in cc mode.It does not show values unfortunately. Not
realy sure how to calculate them.
It is basically just another resistor and cap to create another pole
,but at what frequency.
This is where my ignorance on the subject becomes sorely apparent!!
Bit more experimenting required.
The circuit is basically running quite well. I can deliver up to 150W(
have not pushed it any further)
with a fan heeping things cool.The switching tranny and diode are only
on small heatsinks (20 *30 mm with 2 forward facing
sections of about 10*30mm)
As I said , bit more experimenting required :0>

I am a bit rusty on the compensation, but the gist of the problem is this:

When in discontinuous mode, the supply can respond to a change in
error in the very next power cycle, because each one is completely
independent of the last. All the energy stored in the inductor gets
dumped to the output, each cycle. If more power is needed for the
next cycle, the on time is increased, the energy goes up, and that
larger energy comes out of the rectifier by the end of the cycle.

In CCM, there is energy left in the core when the next power pulse
interrupts the output diode current. So the consecutive cycles are
not independent of each other. In order to raise the output current
(in response to error) the input duty cycle rises. But the effect of
this on the first cycle after the increase is both an increase in the
diode current (during the discharge part of the cycle) and also a
decrease in the conduction time available for that current. For the
first few cycles, this results in a net decrease in average DC current
out. The longer the drive time is boosted, the more severe the output
current drop. Eventually (many cycles later) the stored energy gets
so high that the shorter duration output pulses finally get strong
enough that the average current increases enough to start bringing the
error down. At that point, the charge up time starts to back off.
The initial reaction to this is that the very high output current
lasts longer (all that is left of) each cycle. So the output current
rises as the input duty cycle falls, till the stored energy in the
transformer gets dumped down.

This reverse initial reaction is the difficulty called a response zero
that complicates the control. It requires a loop compensation that
fixes any problem more slowly so that the reverse acting initial
response is swept under the rug. Sometimes, having more output
capacitance helps get by with this slower control.

This same effect takes place when you steer a bicycle. In order to
turn left, you have to first, briefly steer right to move the line of
tire contact with the pavement to your right, causing the bike to be
leaning left. Then you can turn. This limits how fast you can begin
a turn.

Also, when changing the horsepower out of an internal combustion
engine, if you slap the gas pedal down when the engine is idling, you
risk stalling the engine with the power consuming compression stroke
that must precede the increased power producing power stroke. You
hide this problem by accelerating more slowly, or add a bigger fly
wheel to the crank (which also slows acceleration but hides this problem).

 

[[email protected]]

I agree. It lowers the RMS winding current relative to the DC output
current and in some ways, lowers the noise the supply emits. It also
has some negatives, like slower control response to output error and
rougher turn on transient.


I am a bit rusty on the compensation, but the gist of the problem is this:

When in discontinuous mode, the supply can respond to a change in
error in the very next power cycle, because each one is completely
independent of the last. All the energy stored in the inductor gets
dumped to the output, each cycle. If more power is needed for the
next cycle, the on time is increased, the energy goes up, and that
larger energy comes out of the rectifier by the end of the cycle.

In CCM, there is energy left in the core when the next power pulse
interrupts the output diode current. So the consecutive cycles are
not independent of each other. In order to raise the output current
(in response to error) the input duty cycle rises. But the effect of
this on the first cycle after the increase is both an increase in the
diode current (during the discharge part of the cycle) and also a
decrease in the conduction time available for that current. For the
first few cycles, this results in a net decrease in average DC current
out. The longer the drive time is boosted, the more severe the output
current drop. Eventually (many cycles later) the stored energy gets
so high that the shorter duration output pulses finally get strong
enough that the average current increases enough to start bringing the
error down. At that point, the charge up time starts to back off.
The initial reaction to this is that the very high output current
lasts longer (all that is left of) each cycle. So the output current
rises as the input duty cycle falls, till the stored energy in the
transformer gets dumped down.

This reverse initial reaction is the difficulty called a response zero
that complicates the control. It requires a loop compensation that
fixes any problem more slowly so that the reverse acting initial
response is swept under the rug. Sometimes, having more output
capacitance helps get by with this slower control.

This same effect takes place when you steer a bicycle. In order to
turn left, you have to first, briefly steer right to move the line of
tire contact with the pavement to your right, causing the bike to be
leaning left. Then you can turn. This limits how fast you can begin
a turn.

Also, when changing the horsepower out of an internal combustion
engine, if you slap the gas pedal down when the engine is idling, you
risk stalling the engine with the power consuming compression stroke
that must precede the increased power producing power stroke. You
hide this problem by accelerating more slowly, or add a bigger fly
wheel to the crank (which also slows acceleration but hides this problem).

Hi there.
Thanks for the explanation of what is happening in cc mode. Really
helps a lot.
Still lots of studying to be done on my part though :0(
I assume I have to add a R & C to slow response down a bit. My maths is
not so hot anymore ( never was I suppose) , so a bit of trial and error
experimentation is in order.
Maybe if I can find some app notes with this kind of cct on the net
that should point me in the right direction.
Cheers for now.
Rob

 

[John Popelish]

Thanks for the explanation of what is happening in cc mode. Really
helps a lot. (snip)
Maybe if I can find some app notes with this kind of cct on the net
that should point me in the right direction.

I haven't studied this, but a quick scan looks pretty promising.
http://encon.fke.utm.my/nikd/Dc_dc_converter/TI-SEM/slup113.pdf
It took me a couple tries before it came up without timing out.

 

[John Popelish]

Maybe if I can find some app notes with this kind of cct on the net
that should point me in the right direction.

A similar one:
http://focus.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slup084&fileType=pdf