# Temperature rises as duty cycle decreases?



## ngrimm (Oct 19, 2007)

I have been testing a small diy 12 volt pwm on a resistive load and what I am seeing is that the mosfet temperature is cooler as it "approaches" 100% duty cycle. I assumed it would run hotter as the duty cycle and therefore amperage was increased. I am using four mosfets rated at 62 amps each and the total amp draw at nearly 100% duty cycle was 50 amps. Freq = 1.3 Khz and am using 10 ohm gate resistors and a 1000u low esr cap. So what am I missing?


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## Tesseract (Sep 27, 2008)

ngrimm said:


> I have been testing a small diy 12 volt pwm on a resistive load and what I am seeing is that the mosfet temperature is cooler as it "approaches" 100% duty cycle. I assumed it would run hotter as the duty cycle and therefore amperage was increased. I am using four mosfets rated at 62 amps each and the total amp draw at nearly 100% duty cycle was 50 amps. Freq = 1.3 Khz and am using 10 ohm gate resistors and a 1000u low esr cap. So what am I missing?


Switching losses. Though, to be honest, at 1.3khz and 12V it is unlikely you are incurring much in the way of such unless the gate drive signal looks more like a trapezoid than a square wave. Are the flanks of the gate drive pulse steep and hitting at least 10V without any overshoot above 15V? How about the output waveform - a sharp square wave as well? I ask because at very low duty cycles you may not be turning the MOSFETs fully on.


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## major (Apr 4, 2008)

ngrimm said:


> I have been testing a small diy 12 volt pwm on a resistive load


Hey Tesseract,

"on a resistive load". You think that might be part of his problem? No output inductor! Shorter duty cycle, more ripple. And, probably a stupid question but I've seen it forgotten before, got a freewheeling diode?

You've been butting in on those motor questions, so I thought I'd return the favor 

major


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## ngrimm (Oct 19, 2007)

Yeah I have two freewheel diodes on it though they're only rated 40 amps each versus 62 amps per mosfet x 4. I didn't think I needed as many diodes on a non inductive load. They are on the same heat sink so maybe they are getting warmer at lower duty cycle? The most I am driving is 50 amps. The nature of the load is that the resistance decreases as it gets warmer (electrolyser) so the pwm's current limiting attempts to keep the amps constant by reducing the duty cycle and that is when the mosfets get warmer. If I crank the amps up higher at that point, the mosfets run cooler. Just wasn't what I expected.


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## Tesseract (Sep 27, 2008)

Curiouser and curiouser.... Even when your load is "resistive" there's always some inductance present and when you are switching more than a few amps you ought to use a FWD to clamp that energy to the supply, but I wouldn't expect the switches here to get hotter at lower duty cycles UNLESS they were avalanching OR being driven too slowly.

I hear ya, major., I'll try to do a better job staying on my side of the sandbox...


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## Amberwolf (May 29, 2009)

Ngrimm, is it a half bridge (2 quadrant), or just a simple single-switch (1 quadrant) output stage? I'm assuming for the moment that it's 1 quadrant, with the freewheel diodes making up what would have been the other half of a half-bridge. (this is how my old ScootNGo controller was made).

Do the freewheel diodes heat up at all? I suspect not, since the inductance involved to generate spikes is a lot lower than it would be with a motor. But if they do, do they heat up the same way the MOSFETs do or only the opposite way?

I think that it is likely your gate drive is not switching on hard enough. What is your gate drive waveform like? Do you have an oscilloscope you can check it with?

What frequency are you switching at? The more often you switch them, the more power is dissipated within them as heat, because they go thru their linear (higher resistance) phase more often.

What is your gate drive voltage? Most MOSFETs turn fully on at around 10V, though 4V is enough to be virtually completely on. The harder and faster you turn them on, the less power is dissipated within them as heat. So a higher gate drive voltage is generally better. However, it is not JUST voltage that turns them on, they have to get a total amount of charge (to "fill the gate capacitance", if you like) to be able to fully turn on. When they're not fully on, they have higher resistance and have to dissipate more power inside them. It's why so many high-wattage 1970's "solid state" hi-fi amps using MOSFETs in their linear mode blew up so many of the MOSFETs, under high loads, when crammed into small cabinets with no real ventilation. 

What is the circuit you are using to drive them with? Direct output from a PWM chip or a microcontroller? Or a MOSFET driver chip (or equivalent transistor setup)? The PWM chip or microcontroller might only be supplying 5V, or potentially only 3V, which won't charge the gate fast enough to get the MOSFET out of it's linear area quick enough. Also, often they can't sink enough current to discharge the gate fast enough to turn it off sharply, and again the MOSFET stays in it's linear region too long, and heats up. Even with a 10-ohm resistor, there may still be too much charge time. 

The driver chips or push-pull transistor setups will turn the gates on more quickly. 

A full schematic might help diagnose the problem.


BTW, a nice resource for learning a lot of interesting info about PWM controllers is the technical site for 4QD, at http://4qdtec.co.uk 
It's where I started out learning much of what I know now, along with appnotes from various manufacturers about their PWM chips and whatnot. Plus some experience building, blowing up, and repairing my own small controllers. 
________
ROLL A JOINT


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