SG3524 buck converter

[noname]

i am using sg3524 to do my buck converter. i use ir2117 to drive my high side power mosfet (irf540). can anyone help me to check my open loop test circuit. to obtain duty cycle more than 50% pin11 & 14 were shorted is it? how about pin12 & 13?

View attachment 40316

 

[mvs sarma]

Hi Noname,

i have doubt on, whether bottom point of R6 tobe connected ground or to the Rt and Ct combined.  In another open loop test circuit, i observed that Rt and Ct are not shorted.

Also please check the collectors at 12 and 13 (looped) is referes as input to IR2117 while emitters(looped) is referred to the load point , ie source of IRF540 ?
thus so you expect out put at source of is to be sensed when irf54o is off  as a low signal to IR2117?

the IR2117 in point has a schemit inveter followed by Under voltage sensor. How do you expect low(IRF540 OFF time) tobe able to be sent via the NPN transistors inside 3524? perhaps collectors  may go to souce  pin and emmitters go as input to IR2117?  i am not sure but appears logical.

can you give any known ref to this schematic ?
Sarma

 

[noname]

ya the RT & CT are not shorted, is my careless. thanks 4 ur remind.
because i am using n channel mosfet then the emiter pin should go ir2117 input; if use p channel then is collector go the input pin right?
is it correct nw?

View attachment 40318

 

[noname]

here is the test circuit i refer from the datasheet

View attachment 40319

 

[mvs sarma]

Hi Noname,

In amother TI application note  "SLUS180E–NOVEMBER 1999–REVISED OCTOBER 2005", i find that R6 is not commoned . current sense pot lower portion is dirctely grounded.

Next i find that the output at souce of IRF540 is to be taken with some seriies resistor to guard the un committed transistors inside 3524.

then you may perhaps test  in real time.

 

[indulis]

In "your" circuit, you have pins 12 & 13 tied to the drain of the MOSFET... I think they should be tied to either Vref or to Vin unless in your final design the Vin voltage will be to high for MOSTFET driver.

While you could use this voltage mode control PWM (SG3524), it's considered "old" in the industryand there are better PWM's out there that are specifically made for Buck converters (for both synchronous and non-synchronous configurations) and won't need a driver IC, as bootstrap circuitry is onboard for driving high side FET's.

Can you tell us a little more about your design??  i.e. input voltage, output voltage, switching frequency, expected ripple current, output ripple voltage and since this is a voltage mode controller the minimum load current (this will directly impact the inductor value)... etc.

Your IRF540 is big time overkill for a 15V input voltage... is it needed or is it something you just happen to have on hand?

 

[noname]

Thanks a lot. I knw sg3524 is for voltage mode control. Actually i would like to use average current mode control as my buck converter feedback control. But i hav no experience in doing electronic project, i think it is better to start familiar with the PWM first so my friend recomend me try to use this sg3524.

What u means 'pins 12&13 tied to drain'??
What difference if i connect it to Vin?

My converter specification:
input: 10-15V
output: 5V
switching freqquency: 100kHz
output ripple: 1%

As u say bootstrap circuitry is onboard, can u please kindly suggest me which PWM it is?

 

[indulis]

In your schematic you show pins 12 & 13 connected to the drain of the the IFR 540. This is a switching node (the voltage goes up and down and you don't want that). They (pins 12 & 13) should be connected to either Vin or the Vref pin and you need a pull down resistor on the driver input.

First, you can't do current control with this PWM. To do "average" current mode control you will need to sum the high side FET's current information with the flywheel current information and send that back to the PWM (current mode) ramp pin.

Check out

http://focus.ti.com/lit/ml/slup091/slup091.pdf


There are a ton of PWM's from Linear

http://www.linear.com/pc/viewCategory.do?navId=H0,C1,C1003,C1042

but their site seems to be having problems right now, so I can't look one up.

How much output current do you want? It will matter for your inductor value and the output cap ESR!! 50mV of ripple at a 1A output is VERY different from 50mV of ripple at 10A!!