Archive for February 28th, 2012
The MOSFET resistance tester I’m twiddling up for my next column will hold the transistor-under-test at a more-or-less constant temperature using a PWM-controlled Peltier module. The Peltier driver looks like this:
The overall idea is that the relay selects heating or cooling and the MOSFET PWM adjusts the power to keep the module at the right temperature. The feedback comes from a thermistor epoxied to the aluminum block holding the MOSFET, which in turn is epoxied to the module and then to a CPU cooler with a fan. More on that later…
Those fat lines mark the high-current paths: 3.3 A with a 5 V supply, as this Peltier module has about 1.5 Ω resistance. Some early tests show the resulting 17 W can pump the test block down to at least 5 °C and up to at least 40 °C in a few tens of seconds, even without any significant PI (no D) loop tweaking.
When I fired it up a test program that just cycles the PWM up and down, the green LED lit up properly in cooling mode, but the red LED also glowed dimly. Probing the drain showed this nasty ringing when the IRLZ14 MOSFET turned off:
The initial spike happens when the drain current pushes the MOSFET body diode into reverse breakdown at about 70 V (off scale high in the image). The drain goes slightly negative for the next half-cycle as the diode slams into forward conduction, then the energy engages in some serious 5 MHz ringing while it dissipates in the Peltier’s resistance.
Obviously, this is a job for an RC snubber…
A bit of fiddling revealed that a 1.5 nF cap dropped the ringing to 2.8 MHz and a 2.5 nF cap put it at 2.4 MHz:
Notice that just putting a capacitor across the MOSFET doesn’t reduce the ringing. What’s needed here is some additional energy dissipation.
Splitting the difference says 2.3 nF would reduce the resonant frequency by a factor of 2, so the original stray capacitance is about (2.3 nF / 3) = 770 pF.
Knowing the resonant frequency and stray capacitance, the stray inductance falls out:
L = 1/[(2∏ 2.5x106)2 770x10-12] = 5.4x10-6 = 5.4 µH
The Peltier module doesn’t have nearly that much inductance, so it’s hidden in the wiring and relays.
Knowing L and C, the characteristic impedance of the circuit is:
Z = √(L/C) = 84 Ω
The snubber cap should be at least a factor of 4 larger than the stray capacitance, which gives 3 nF. Some rummaging produced a small 3.9 nF 100 V Mylar cap (measuring 3.7 nF, close enough) and an 82 Ω resistor, which gave this pleasing result when soldered across the MOSFET source & drain:
The upper trace shows a pair of 32 kHz PWM pulses. The lower trace gives a magnified view of one pulse; the peak remains at about 70 V just after turn-off, because that 3.3 A must go somewhere: that’s why MOSFETs have husky body diodes with reverse-breakdown specs.
A better view of the snubbed peak shows it’s all over in about 400 ns:
The lower trace is the MOSFET gate drive pulse at the Arduino pin, showing the Miller capacitance delaying the transition. It turns out that removing the 22 Ω gate damping resistor doesn’t improve things, but, given the speed of the transition, I think it’s good enough.
The MOSFET burns at (3.3 A × 70 V) = 230 W during that 100 ns peak, which works out to a mere 23 µJ (assuming constant current, which isn’t the case). The IRLZ14 has a 40 mJ single-pulse rating, so it’s in good shape.
The DC dissipation is (3.3 A)2 x 20 mΩ = 2 W: the huge heatsink I stuck on the MOSFET doesn’t have a chance to get warm during the short tests so far.
The red LED remains dimly lit, which goes to show how sensitive a human eye can be: the negative transient is barely 100 ns long!