smps problem

[ksoner]

Hi;

hope the questions is relevant with the group:

I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
The topology is flyback and I decide the output(27V or 13V) with two
jumpers. the problem is that when I switch from 13.8 to 27.6V VCC also
increases ()and I dont want VCC to increase above 30V because the FPS
of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
to limit the current of zener on VCC circuitry. but those dissipate too
much power and zener live on the limits. Is there a nice method(fast
and cheap) to limit the voltage on VCC rather than using a
zener+resistor. thanks in advance

 

[Genome]

Hi;

hope the questions is relevant with the group:

I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
The topology is flyback and I decide the output(27V or 13V) with two
jumpers. the problem is that when I switch from 13.8 to 27.6V VCC also
increases ()and I dont want VCC to increase above 30V because the FPS
of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
to limit the current of zener on VCC circuitry. but those dissipate too
much power and zener live on the limits. Is there a nice method(fast
and cheap) to limit the voltage on VCC rather than using a
zener+resistor. thanks in advance

Oooops...... programmable power supply with auxilliary taken from the main
transformer. Bonk!

Depending on how you define fast and cheap then you might have to split the
auxilliary winding on your tranformer in two and connect the taps as
required.....

DNA

 

[Ken Smith]

Hi;

hope the questions is relevant with the group:

I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
The topology is flyback and I decide the output(27V or 13V) with two
jumpers. the problem is that when I switch from 13.8 to 27.6V VCC also
increases ()and I dont want VCC to increase above 30V because the FPS
of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
to limit the current of zener on VCC circuitry. but those dissipate too
much power and zener live on the limits. Is there a nice method(fast
and cheap) to limit the voltage on VCC rather than using a
zener+resistor. thanks in advance

How much does the input voltage vary? During the "on time" the
transformer has the input voltage for both modes. If you invert the
connections of the AUX winding, the AUX supply will be unregulated but not
change when you change output voltages.

or:

You could make a low powered zener regulator and use a highish voltage
MOSFET as a source follower.


or:

I throw this circuit out there for folks to have fun with:

ASCII art

C1
-------!!-------
! D2 ! D3
! --->!--+-->!--+--Vb
! D1 ! !
Xformer ---+-->!----+ ---
! --- C2
R1 !!- !
Vb -/\/\-+---+-----!! Q1 GND
! ! !!-
D4/-/ c\! !
^ !-/\/\-+---+----------- Vout To load
! e/! R2 !
Vout ! Q2 ---
/-/D5 --- C3
^ !
! GND
GND


D1 and Q1 have to be modestly large.

C1, D2, D3 makes C2 have a highish voltage on it that is always there.

Q1's drain gets its power from the unfiltered diode D1 and thus can only
charge C3 during the flyback time. If Vout tries to go more than a diode
drop above D5's rating, Q2 yanks the gate of Q1 down to prevent this.

During the primary side's on time, the load discharges C3 slighty turning
Q2 off and allowing the gate on Q1 to rise until, perhaps, D4 limits it.

At the instant of primary turn off, Q1 is biased on. The transformer flies
back until D1 starts feeding current to C3. The transformer's swing
pauses at that point until Q2 shuts Q1 off.