100kHz SMPS control

[Jamie Morken]

Hi,

I was initially thinking of designing a 100kHz SMPS with using 200kHz
ADC feedback of the output voltage and current fed into a DSP which
would directly interface to the fet drivers, and using cycle by cycle
regulation based on these readings, but I think this may be very
difficult to read these signals cleanly without them being noisy
from all of the switching noise. So as an alternative I am thinking
that it would be better to use a SMPS controller IC with voltage and
current feeback inputs, which seem to be easier to get to work than a
200kHz ADC in this environment and use its output PWM signal as an
input into a DSP or CPLD/FPGA for controlling the primary side
fullbridge to drive the transformer. This reduces the flexibility of
the power supply as the current and voltage limits are set in hardware
rather than software, unless some DAC/comparator etc setup is used to
drive the SMPS controller IC feedback, but it is easier to trust a
SMPS controller IC rather than a complex ADC/DSP software system to
control a power supply. Perhaps a tradeoff would be a controller IC
with a digital interface to allow on the fly adjustment of the
voltage and current limits as well as the PWM frequency etc. Does an
IC like this exist? I think it would be pretty useful! Any people
here ever use high speed ADC and software to control an SMPS?

cheers,
Jamie

 

[Jan Panteltje]

Hi,

I was initially thinking of designing a 100kHz SMPS with using 200kHz
ADC feedback of the output voltage and current fed into a DSP which
would directly interface to the fet drivers, and using cycle by cycle
regulation based on these readings, but I think this may be very
difficult to read these signals cleanly without them being noisy
from all of the switching noise.

You would need to sample much faster, if you want to monitor
for example the rising current in an output switch, so you
can switch off if it exceeds some level, you need to sample at at least
(estimate) 20x during the max on time, say 10MHz would be OK perhaps.


i /------------ I max, max allowed current in switch
/
/
/
/
/
/ t
0 5 us

.......
sample points

It is then much simpler to use an analog comparator at I max on
an input pin of the micro.

Given the above limit protection working, of course your control loopp
sampling could be slower, but if you wanted fast control of current limit
you would need this high speed again.
That could then be done by setting the comparator reference.

The main issue in the above is that you want to protect your switches.

 

[Nico Coesel]

You would need to sample much faster, if you want to monitor
for example the rising current in an output switch, so you
can switch off if it exceeds some level, you need to sample at at least
(estimate) 20x during the max on time, say 10MHz would be OK perhaps.


i /------------ I max, max allowed current in switch
/
/
/
/
/
/ t
0 5 us

.......
sample points

It is then much simpler to use an analog comparator at I max on
an input pin of the micro.

That is not really necessary. Sampling once at the end of the on-time
is enough. The input voltage is known and the inductor is known. From
this you can calculate a maximum on-time that will not destroy the
inductor or the switch.

To limit the output current, you can measure the voltage across a
sense resistor or the MOSFET. I recently built a circuit based on this
method. To sense the MOSFET current I use an RC timing constant which
is discharged through a diode into the MOSFET. The voltage across the
capacitor is the diode forward voltage + the MOSFET across the MOSFET.

 

[Jan Panteltje]

That is not really necessary. Sampling once at the end of the on-time
is enough.

It seems to me that, if due to some fault condition (could be load, or
whatever), it is possible the current would rise much faster, and the
device would destruct before you even sample (so before the end of
the on time).

The input voltage is known and the inductor is known. From
this you can calculate a maximum on-time that will not destroy the
inductor or the switch.

Eh, that depends on the configuration.
If L is transformed back from the load, then the L you see can become
much smaller, think shorted output for example.

That actually reminds me, we had this guy come in and demonstrate UPSs.
LOL
We did blow it up with a short.

To limit the output current, you can measure the voltage across a
sense resistor or the MOSFET.

I did use a sense resistor in series with the bottom side of the half
bridge in one design.
But I used a comparator.
Note that PIC s often have a build in analog comparator one could use.
That makes things a lot easier and reliable.

I recently built a circuit based on this
method. To sense the MOSFET current I use an RC timing constant which
is discharged through a diode into the MOSFET. The voltage across the
capacitor is the diode forward voltage + the MOSFET across the MOSFET.

Clever. This should work for reasonable values of Ron I suppose?

 

[MooseFET]

Hi,

I was initially thinking of designing a 100kHz SMPS with using 200kHz
ADC feedback of the output voltage and current fed into a DSP which
would directly interface to the fet drivers, and using cycle by cycle
regulation based on these readings,

Don't do it. That way lies madness!

You should initially be thinking about what exactly you are trying to
do that can't be done with a much-much simpler chip than a DSP.

If you are using a DSP or a PIC or anything like that you need to
protect the system during the time that the part takes to wake up and
start working. For development and likely also in production, you
need to add hardware to keep things safe if the code goes wandering
off into the weeds. This usually ends up as complex as just using a
switcher chip and controling it with the DSP or micro.

but I think this may be very
difficult to read these signals cleanly without them being noisy
from all of the switching noise.

If you want high frequency information the spikes are a big problem.
One thing that can work for you is that you can trip the converter at
a time when nothing is switching.

So as an alternative I am thinking
that it would be better to use a SMPS controller IC with voltage and
current feeback inputs, which seem to be easier to get to work than a
200kHz ADC in this environment and use its output PWM signal as an
input into a DSP or CPLD/FPGA for controlling the primary side
fullbridge to drive the transformer.

Don't put any programable part in the important path of the
converter. Use a PWM controller and the like for the whole loop and
then use the DSP to command the controller what to do. If you need a
little more logic in the path use real logic gates or perhaps a
22V10. Any logic more complex that that should have you questioning
what you are doing. I have found that most 22V10s wake up sane. Very
few FPGA or CPLDs work right before they get their morning coffee.

This reduces the flexibility of
the power supply as the current and voltage limits are set in hardware
rather than software, unless some DAC/comparator etc setup is used to
drive the SMPS controller IC feedback,

Chances are you don't need the comparitor. You can set the important
points with DACs and op-amps. A portion of the proportional feedback
can be done by resistors on the PWM chip. The integral feedback can
be done inside the DSP or micro.

but it is easier to trust a

SMPS controller IC rather than a complex ADC/DSP software system to
control a power supply. Perhaps a tradeoff would be a controller IC
with a digital interface to allow on the fly adjustment of the
voltage and current limits as well as the PWM frequency etc. Does an
IC like this exist? I think it would be pretty useful! Any people
here ever use high speed ADC and software to control an SMPS?

Do you mean successfully?

 

[Nico Coesel]

Don't do it. That way lies madness!

You should initially be thinking about what exactly you are trying to
do that can't be done with a much-much simpler chip than a DSP.

If you are using a DSP or a PIC or anything like that you need to
protect the system during the time that the part takes to wake up and
start working. For development and likely also in production, you
need to add hardware to keep things safe if the code goes wandering
off into the weeds. This usually ends up as complex as just using a
switcher chip and controling it with the DSP or micro.

Don't over react please Software PWM controllers can be major cost
savers. A 200kHz PWM converter costs a few bucks. A PWM output on a
controller usually comes for free. If the PWM is controlled using a
high priority interrupt, the chance of things going wrong is very
small.

 

[Tim Wescott]

Hi,

I was initially thinking of designing a 100kHz SMPS with using 200kHz
ADC feedback of the output voltage and current fed into a DSP which
would directly interface to the fet drivers, and using cycle by cycle
regulation based on these readings, but I think this may be very
difficult to read these signals cleanly without them being noisy from
all of the switching noise. So as an alternative I am thinking that it
would be better to use a SMPS controller IC with voltage and current
feeback inputs, which seem to be easier to get to work than a 200kHz ADC
in this environment and use its output PWM signal as an input into a
DSP or CPLD/FPGA for controlling the primary side fullbridge to drive
the transformer. This reduces the flexibility of the power supply as
the current and voltage limits are set in hardware rather than software,
unless some DAC/comparator etc setup is used to drive the SMPS
controller IC feedback, but it is easier to trust a SMPS controller IC
rather than a complex ADC/DSP software system to control a power supply.
Perhaps a tradeoff would be a controller IC with a digital interface to
allow on the fly adjustment of the voltage and current limits as well as
the PWM frequency etc. Does an IC like this exist? I think it would be
pretty useful! Any people here ever use high speed ADC and software to
control an SMPS?

cheers,
Jamie

Microchip is pushing the idea of using their chips as SMPS controllers --
why don't you check their web site for app notes?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[MooseFET]

Don't over react please Software PWM controllers can be major cost
savers. A 200kHz PWM converter costs a few bucks. A PWM output on a
controller usually comes for free. If the PWM is controlled using a
high priority interrupt, the chance of things going wrong is very
small.

I don't see it as over reaction. Spraying the entire lab with flaming
capacitor parts and the epoxy that used to be the package for the
power semiconductors is a joy I wouldn't deny the OP. On the other
hand in these green days, blast shields with a asbestos coverings are
hard to get.

 

[legg]

Don't over react please Software PWM controllers can be major cost
savers. A 200kHz PWM converter costs a few bucks. A PWM output on a
controller usually comes for free. If the PWM is controlled using a
high priority interrupt, the chance of things going wrong is very
small.

In my experience, the 'cost savings' are achieved only in avoiding
problems that originate in the application or programming of the DSP
or microcontroller.

The versatility of the DSP / microcontroller has to be weighed against
all of the nit picky add-ons required to ensure that misbehavior in
the silicon, interconnection or code doesn't result in a
'nonrecoverable' event.

My preference is that these devices be used in such a way that they
can only modify normal safe operation, as may be desired Their failure
should not affect the ability of the system to continue in operation -
their failure, if detectable, should merely produce a fault warning
that can be addressed separately, in a safe manner, by other
intelligence.

Having seen enough of them converted to mush, the first time a simple
fault occurs, these things are preferably socketed, or isolated on an
FRU. The 'other intelligence', by the way, should not be configured
with similar shortcomings.

CPLDs, on the other hand, have the potential to be considered as
clock-work glue logic, should they ever be produced in a sufficiently
simple and rugged format and not misapplied. At present they seem to
exhibit the same supply fault dependencies and require similar
precautions in their use.

RL

 

[Tam/WB2TT]

Hi,

I was initially thinking of designing a 100kHz SMPS with using 200kHz
ADC feedback of the output voltage and current fed into a DSP which
would directly interface to the fet drivers, and using cycle by cycle
regulation based on these readings, but I think this may be very
difficult to read these signals cleanly without them being noisy
from all of the switching noise. So as an alternative I am thinking
that it would be better to use a SMPS controller IC with voltage and
current feeback inputs, which seem to be easier to get to work than a
200kHz ADC in this environment and use its output PWM signal as an
input into a DSP or CPLD/FPGA for controlling the primary side
fullbridge to drive the transformer. This reduces the flexibility of
the power supply as the current and voltage limits are set in hardware
rather than software, unless some DAC/comparator etc setup is used to
drive the SMPS controller IC feedback, but it is easier to trust a
SMPS controller IC rather than a complex ADC/DSP software system to
control a power supply. Perhaps a tradeoff would be a controller IC
with a digital interface to allow on the fly adjustment of the
voltage and current limits as well as the PWM frequency etc. Does an
IC like this exist? I think it would be pretty useful! Any people
here ever use high speed ADC and software to control an SMPS?

cheers,
Jamie

You have to be careful doing that. We had to redesign a piece of gear that
used the system 8051 to control a -48 to +5 converter. Blew about 6
transistors before the software guy got his code right. It would be
worthwhile to have some hardware gating to prevent doing something real
stupid.

Tam

 

[Jamie Morken]

That is not really necessary. Sampling once at the end of the on-time
is enough. The input voltage is known and the inductor is known. From
this you can calculate a maximum on-time that will not destroy the
inductor or the switch.

The 100kHz transformer I have only has 50uH primary inductance with
500VDC what would the maximum on time be for it?

cheers,
Jamie

 

[MooseFET]

You have to be careful doing that. We had to redesign a piece of gear that
used the system 8051 to control a -48 to +5 converter. Blew about 6
transistors before the software guy got his code right. It would be
worthwhile to have some hardware gating to prevent doing something real
stupid.

...and that was on an 8051 which is very easy to code for. It wasn't a
DSP or a PIC which can be a lot harder to code on.

I am sort of surprised that the software guy didn't dream up some way
to fully test his code in a more protected environment.

 

[Jamie Morken]

Microchip is pushing the idea of using their chips as SMPS controllers --
why don't you check their web site for app notes?

I guess that is a dsPIC with an analog comparator?

cheers,
Jamie

 

[Nico Coesel]

..and that was on an 8051 which is very easy to code for. It wasn't a
DSP or a PIC which can be a lot harder to code on.

Doesn't matter for a real embedded software engineer.

I am sort of surprised that the software guy didn't dream up some way
to fully test his code in a more protected environment.

Now you are making sense. Test first then try in a real-life circuit.
Still, software like this should be written in a way the output is
limited by some hard boundaries based on the hardware. So when
calculations go wrong, the output isn't going beyond the limits of the
hardware.

 

[Jamie Morken]

To limit the output current, you can measure the voltage across a
sense resistor or the MOSFET. I recently built a circuit based on this
method. To sense the MOSFET current I use an RC timing constant which
is discharged through a diode into the MOSFET. The voltage across the
capacitor is the diode forward voltage + the MOSFET across the MOSFET.

I don't understand how that is wired up, could you elaborate on the
circuit? The current sense I've seen for SMPS controller IC's use
a sense resistor and an RC lowpass filter fed into the Isense pin
of the controller IC.

cheers,
Jamie

 

[Nico Coesel]

I don't understand how that is wired up, could you elaborate on the
circuit? The current sense I've seen for SMPS controller IC's use
a sense resistor and an RC lowpass filter fed into the Isense pin
of the controller IC.

My application is a step-up converter so the MOSFET switches to
ground. I've connected a resistor and capacitor in series between 3v3
and ground (resistor is connected to 3v3). Now a diode is connected
between the drain of the mosfet and the capacitor. Every time the
MOSFET switches on, the capacitor is discharged but not further than
voltage accros the MOSFET + the diode forward voltage. In other words,
the voltage across the capacitor is a (crude) measure for the amount
of current flowing through the MOSFET.

 

[Tam/WB2TT]

Doesn't matter for a real embedded software engineer.


Now you are making sense. Test first then try in a real-life circuit.
Still, software like this should be written in a way the output is
limited by some hard boundaries based on the hardware. So when
calculations go wrong, the output isn't going beyond the limits of the
hardware.

Where the guy went wrong was in turning the OFF transistor ON before turning
the ON transistor OFF. They lasted a few minutes. Not obvious why he messed
with that part of the code at all; possibly to replace assembly code with c.

Tam

 

[Jamie Morken]

My application is a step-up converter so the MOSFET switches to
ground. I've connected a resistor and capacitor in series between 3v3
and ground (resistor is connected to 3v3). Now a diode is connected
between the drain of the mosfet and the capacitor. Every time the
MOSFET switches on, the capacitor is discharged but not further than
voltage accros the MOSFET + the diode forward voltage. In other words,
the voltage across the capacitor is a (crude) measure for the amount
of current flowing through the MOSFET.

Does this circuit have any advantages to using a sense resistor below
the mosfet feeding an RC lowpass? Besides the trade between a diode
and a sense resistor are they pretty well the same performance?

cheers,
Jamie

 

[MooseFET]

Where the guy went wrong was in turning the OFF transistor ON before turning
the ON transistor OFF. They lasted a few minutes. Not obvious why he messed
with that part of the code at all; possibly to replace assembly code with c.

That, I think, explains it. Many people somehow think that code in a
low level language that works should be replaced with code in a higher
level language.

I still think that it is surprising that he put code that did that
onto real hardware. I often use a scope in checking and debugging my
code that works with low level hardware. I also tend to build in
interlocks so that things like attempts to turn on both transistors
forces things into a safe state. It usually takes very little extra
to do it.

Then again with no micros involved, we just blew something up in the
lab by turning a knob. We applied about 60V oooops!.

 

[MooseFET]

Does this circuit have any advantages to using a sense resistor below
the mosfet feeding an RC lowpass? Besides the trade between a diode
and a sense resistor are they pretty well the same performance?

The current measurement this method makes is very crude but in
practice it can be a good way to go. It doesn't consume much power
and gives a measure of the conduction drop of the MOSFET which is a
good measure of how hard it is being forced to work.