Keep it cool – part 2

Driving loads

You may not expect to read about multiple amp loads in the context of “Low Power Design”. It could be an indication of how wide-spread the push for energy efficient products has become.  In many embedded applications it is quite common for a micro drawing a few milliamps to control motors or solenoids that require several amps of current or high-brightness LEDs that draw several hundred milliamps. Even if these current ranges are way above what your design has to deal with, the information presented here may be applicable to your design.

When driving a large load such as a DC motor, solenoid or a cluster of LEDs, there are a number of things you can do to ensure you drive them efficiently to help reduce heat buildup:

  • You already selected the lowest RDSon FET you could find but are you taking full advantage of its capabilities? Make sure your gate voltage is such that the FET is fully turned on and operating in its lowest RDSon range. Most N-channel FETs usually need a gate voltage in the 8-10V range to fully turn on so simply driving the gate with a GPIO won’t put the FET in its lowest RDSon range. Logic level gate FETs may be better in this regard but typically they just have a lower minimum turn-on threshold and may still need over 5V to fully turn on. In these cases you should consider using a P-channel FET or even a gate driver IC to drive the gate of the N-channel FET with the voltage it is switching (or use a step-up regulator or voltage doubler circuit to provide a higher voltage if the input voltage exceeds the maximum gate voltage). The graph below shows the impact of gate voltage on Rds(on) for the Fairchild FDS8449 N-channel FET. The FDS8449 has a max gate turn-on threshold of 3V but as you can see at 3V the Rds(on) is 2.4X higher than at 10V at no load, much higher as the load increases.

Rds(on) vs gate voltage

  • When using a P-channel FET to drive a load, a GPIO may not drive the gate high enough to completely turn off the FET so you may be leaking power through the FET. This can often go un-noticed since the amount of power is too low to activate the load. Using an open-collector driver or an N-channel FET to drive the gate of the P-channel FET can solve this problem (don’t forget a pull-up resistor from the P-channel FET’s gate to the voltage being switched).
  • A P-channel FET of similar rated voltage and current as an N-channel FET will typically have 50-100% higher Rds(on) than the N-channel FET. With Rds(on) specs on modern FETs in the double-digit milliohm range even doubling the Rds(on) produces a fairly low value. However, that is simply wasted power that can easily be eliminated if low-side switching is an option for your application. This is also important to keep in mind if for some reason you can’t address one of the issues discussed here that prevents the FET from operating close to it’s lowest Rds(on), changing the type of FET may alleviate the problem. If you have a P-channel FET operating at 2X its lowest Rds(on) then you could possibly reduce the Rds(on) by a factor of 4X by using an N-channel FET.
  •  Just like a resistor, the Rds(on) of a FET increases with temperature. The graph below shows the impact of temperature on Rds(on) for the Fairchild FDS8449 N-channel FET. As you can see, a 50°C increase in temperature results in a nearly 20% increase in Rds(on). Even if your product is normally used in a room temperature environment, a 20-50°C temperature rise at the FET’s die isn’t uncommon (another reason to operate the FET in its lowest Rds(on) range). This is another situation where keeping a part cooler helps prevent wasting power, as the Rds(on) increases the part will get hotter, increasing the Rds(on) and so on. A hot FET and a non-optimal gate voltage can combine to generate a lot of excess heat and waste a lot of power.

Rds(on) vs junct temp

  • Similar to LEDs, electromechanical devices can often be driven with a PWM to reduce power without impacting the performance of the device. Solenoids can often be “kicked” with a several hundred millisecond pulse to actuate them and then driven at as low as 30 or 40% duty cycle to keep them actuated. Depending on size, a DC motor may require a “kick” for up to a few seconds to get it up to speed and more than a 50% duty cycle to avoid slowing down but with such large loads, even a 10-20% savings can be a considerable reduction in power consumption.

 If your application involves high currents or other potential heat sources, consider buying or renting an infrared thermal camera. The camera will help you find hot spots on your board and access the effectiveness of your heat spreading/isolation efforts. Don’t be tempted to skimp and use a laser pointer IR thermometer instead of the camera. The IR thermometer has a fairly narrow range of view and only displays the temperature where you point it. An IR thermometer may not accurately measure the temperature of a small hot spot like one caused by high current flowing through a small surface mount FET.  The beauty and value of the camera is it can show you even tiny hotspots where you don’t think about looking for them. The first time I used a thermal camera it did just that, allowing us to fix an issue at the prototype stage that would have likely lead to field failures and high warranty costs. That camera more than paid for itself in a matter of hours.


Driving loads