How to create a ramp generator for boost converter control??

[mengghee]

lm339 boost converter

Hi,
I am just wondering if anybody can give me some suggestions on how to create a ramp generator which has a frequency of approximately 50khz-100khz. I am required to build the ramp generator for the control of my boost dc to dc converter. thank you thank you. I have also been told by one of my lecturer that i would not be able to implement a boost with such a high frequency due to the capacitance in the board. is that true ? thanks...

 

[electronrancher]

Re: How to create a ramp generator for boost converter contr

I think this frequency is fine, I build 100kHz boosts on breadboard and they work. There will be some efficiency loss due to charging and discharging the stray capacitances, but I don't think you are going for an efficiency record..

Anyway, here is a common ramp gen I have used. It charges a cap to Vcc/2, then discharges it to 0 and starts again.

Here's how it works - On startup, R1/R2 create a threshold of Vcc/2, and the comparator gives a low output so the cap can charge up. When the cap charges up to Vcc/2, the comparator goes high. This shorts out R2, changing the reference voltage to Vcc/10 and discharges the cap quickly. Once the cap falls below Vcc/10, the comparator goes low and the cap can begin charging again.

Q3/Q4 is a current mirror used to charge the cap with a constant current. The current is (Vcc-0.65)/R7. You could simply use a 10k to Vcc to charge the cap, but the ramp would have a slight curve to it - not a big deal for experimentation, but that curve means that the gain of the PWM modulator would change slightly as the duty cycle changed - you can tell your lecturer that so they think you are a PWM master!

Usually, when I use this circuit to build a boost converter, I use the output of the comparator to force a reset on the PWM flip-flop, giving a nice max duty cycle gate and preventing the loop from locking up in the ON state.

Do you want me to draw the rest of the boost?

 

[electronrancher]

Re: How to create a ramp generator for boost converter contr

OK, I went ahead and finished the schematic. It's set up for conversion from 5v to 12v at about 80kHz. In sim, the efficiency is 89% at a 1A load (12W out).

It uses 3/4 of an LM339 quad comparator, and a handful of discrete devices. Feel free to mess around with any of the blocks. The error amp is pretty crummy, as it's just a gained-down comparator. And the mosfet driver is really crummy, even a bunch of hex inverters in parallel would be better - but at 80kHz you're not really looking for 10 nanosecond risetimes...

Since this circuit is a voltage mode, you'll need a second zero - I created this by a cap across the top FB resistor, but you can use an op amp error amp with type 3 compensation, etc, if you want.

You need to supply a 1v reference to the error amp, but this is pretty much standard in regulators.

Good luck!

 

[mengghee]

Re: How to create a ramp generator for boost converter contr

Thank you very much electronrancher. I will try to simulate that circuit diagram. I am also wondering if It will work if i am using a ramp generator chip such as MAX038 as i am allowed by my lecturer to use a chip. I am also wondering how to calculate efficiency of a boost converter ... thanking you in anticipation