Boost SMPS compensation problem

[treez]

Hi,

I'm working for a company in Chicago and doing continuous mode boost smps 17V in and 27V out to drive LEDs at 350mA for signage.

Anyway:

The following circuit uses components C1, C2 and R5 to give a Type 2 compensaton to the boost:-
.....

Here is the LED current at start up for the above circuit......

...please notice how there is serious overshoot of the current up to 0.9A at switch_on...this is due to the resonant behaviour of the boost's L's and C's at the application of V(in).
Please also notice that later on, the LED current reaches 350mA but first there is overshoot of the LED current up to about 400mA.

These are serious effects, LEDs do not have the surge current ratings of normal diodes and these LEDs will die much quicker than well-treated LEDs.

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To correct for the above effects, a 10uF capacitor C9 (below) is added across the LEDs.
(supplementary diodes D11 and D13 ensure that C9 does not effect the circuit after turn_on ...D13 gives a discharge path for C9 when the boost is switched off)

C9 basically gives a soft start to the boost.....here is the augmented circuit......

and here is the LED current waveform with this added C9......

...Please notice how the resonant surge current in the LEDs, and the start-up overshoot LED current are both now TOTALLY CURED.
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Can readers confirm that that the augmented circuit with C9 provides Type 3 compensation?

Can readers also confirm that the first circuit, with only C1, C2 and R5 as compensation components (no C9), would NEVER be able to cure either the switch_on resonant surge current OR the start-up overshoot LED current. -No matter how the values of C1, C2 and R5 were modified ?

 

[tantrajna]

I think your first problem is that you have not designed a CC supply. Many people don't know that the LEDs with change the current through them DRASTICALLY when the voltage across them changes even a little bit. That's why you design for CC (constant current.)

C9 does not provide type 3 compensation. Sorry. 2 poles and 2 zeroes are not introduced by 1 capacitor. Try increasing C9 to 47uF or 100uF. You may get a better response as softstart cap is increased.

The compensation network you have R5 C1 and C2 is a Type 2. You can probably play with R5 making it 5k or 10k with C1= 0.1uF. Don't make C1 and C2 very close to each other. This way you may get rid of the starting overshoot of about 150mA that you have in the 2nd pic.