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Archive for March 5th, 2012

Peltier PWM Temperature Control: MOSFET Turn-on Time Constant

On the other end of the Peltier driver’s PWM pulse, the MOSFET turns on with a surprisingly lengthy time constant:

Peltier Drain - Turn-On

Peltier Drain - Turn-On

The upper trace shows the drain voltage drops to nearly 0 V as the transistor turns on, then rises to about 600 mV. The IRLZ14 spec says RDS is 200 mΩ max, so that’s in line with the actual 3.3 A through the Peltier module, and puts the dissipation at 2.2 W.

The lower trace is the gate drive, showing a small Miller effect. The Channel 2 scale readout is off by a factor of 10, as I forgot to tell the scope that I was using a 10x probe. It’s really 5 V/div, not 500 mV/div.

The cursors put the time constant at 1.3 µs. If the inductance is the 5.4 µH indicated by the turn-off resonance, then the total circuit resistance is nearly 4 Ω… which is obviously not the case, given the 5 V supply voltage and the 3.3 A current.

Looking at the Peltier module’s power supply at the board terminal reveals the true cause:

Peltier Supply Transient

Peltier Supply Transient

The scale is 1 V/div with 0 V at the bottom of the screen , so the switching supply produces 5.2 V with no load and 4.6 V at about 3 A. It’s rated at 5 V and 3.7 A, so the Peltier current is right up near its limit.

The glitch when the MOSFET turns off shows that the supply can’t absorb much transient power in either direction, which is typical of switching supplies. In this day & age, there’s no bulk capacitance to smooth out line-frequency ripples from a full-bridge rectifier.

The total circuit resistance is about 1.8 Ω, figuring the Peltier module at 1.5  Ω, the MOSFET at 0.2  Ω, and everything else at 0.1  Ω. That says the actual current is around 2.6 A, although the fancy Tek Hall Effect probe I mooched from Eks puts it at almost exactly 3 A; I’d tend to trust the Tek probe’s opinion more than my sum of small numbers. With 4.6 V and 3 A, the total resistance is spot on 1.5  Ω. The Peltier module’s resistance is temperature sensitive, so a few tenths of an ohm variation isn’t entirely unexpected.

So that says the L/R time constant is (5.4 µH / 1.5  Ω) = 3.6 µs, which makes more sense: it’s entirely masked by the power supply transient.

A touch of bulk capacitance may be in order. To supply 3 A for 5 µs  with 0.5 V droop:

C = IΔT/ΔV = 3•5x10-6/0.5 = 30 µF

Well, that’s not so bad after all… I’m sure I have a high-voltage cap along those lines.

There’s a reason the MOSFET tester has connectors for three separate supplies: I expected nasty transients from a high-current PWM load. One supply for the Peltier, another for the MOSFET-under-test’s drain, and a triple-output supply for the Arduino and analog circuitry.

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