N-Channel MOSFET High-Side Drive: When, Why and How?

I had previously shown how to drive N-channel MOSFETs in low-side configuration. You can find the tutorial here:

http://tahmidmc.blogspot.com/2012/12/low-side-mosfet-drive-circuits-and_23.html

I’ve been requested to write a tutorial/article regarding high-side MOSFET drive. So, here I’ll talk about N-channel MOSFET high-side drive.

Let’s first look at the common low side configuration.

 Fig. 1 - N-channel MOSFET configured as low-side switch

Now let’s look at a MOSFET configured as a high-side switch.

Fig. 2 - N-channel MOSFET configured as high-side switch

You can quite easily see the difference between the high-side configuration and the low-side configuration. In the low-side configuration, the load is connected between the drain and +V, while the source is connected to ground. Thus, the gate drive is referenced to ground. So by applying a voltage of >7V (for Power MOSFETs) or >4V (for Logic Level MOSFETs), the MOSFET can be fully turned on.

Now let’s talk about the high-side configuration. The load is connected between the source and ground with the drain connected to +V. Thus the gate drive is not referenced to ground as source is not connected to ground and gate drive is related to VGS (voltage at gate with respect to source). I’ll talk about this in a while.

The difference in operation for the MOSFET configured as high-side switch as opposed to the MOSFET configured as low-side switch is that, it acts as a current source whereas the MOSFET configured as low-side switch acts as a current sink.

Now let’s go back to the high-side drive. Let’s say you apply a voltage of 12V (with reference to ground) to the MOSFET gate. However, when the MOSFET is on, voltage at source is equal to +V. Let’s assume +V is +15V. Now the problem is +12V gate drive (with reference to ground) will not keep the MOSFET on. When the MOSFET is on, the MOSFET source will be at a potential of +15V. To be on, the MOSFET must have +8V VGS minimum. So, if source is at +15V, the voltage at the gate with respect to ground must be at least +23V. If source was at +300V, for example, gate drive would require a minimum of +308V with respect to ground. This is if the gate drive is referenced to ground. If you have a separate isolated power supply whose ground and the ground of the MOSFET-based circuit are isolated, then  you can use that to drive the MOSFET as well.

There are quite a few ways to drive MOSFETs in high-side configuration. The first thing that might come into the minds of many of you would be a boost converter circuit or a charge pump circuit to use as the drive voltage for gate drive. This concept is sometimes used and isn’t wrong. However, it is usually used when voltage gap between control circuit voltage and gate drive requirement is small. If you needed to step up voltage from 12V to 40V for example, you might be able to accomplish it quite easily. However, a problem arises when there is the need to step up voltages from 12V to, say, 300V. In such situations, other solutions must be sought.

One simple solution is to use a gate drive transformer. This is not a method I prefer and thus I will not talk about it here. If you are interested, I might write another article just for gate drive transformers.

Another solution is to use a separate/isolated power supply whose ground is separate from the ground of the MOSFET-based circuit. See Fig. 3 below.

The other popular method is to use bootstrap based drive. In this drive method, a capacitor is charged to the required VGS – let’s say 10V – when the MOSFET is off. Then this capacitor is used during driving the MOSFET to provide an additional 10V over the source.

First I’ll talk about the use of a separate/isolated power supply. Here’s a circuit diagram illustrating this:

Fig. 3 - Driving a high-side N-channel MOSFET from a separate/isolated power supply (click image to enlarge)

When a logic high is given to “Drive Signal”, potential at optocoupler pin 4 (emitter of optocoupler transistor) is about +12V with respect to the ground / negative terminal/point of BAT1 – the separate/isolated power supply. This point is connected to Q1 source. Thus Q2 turns on. About +12V, with respect to Q1 source, is provided to Q1 gate. Thus Q1 is driven on.

When a logic low is given to “Drive Signal”, optocoupler pin 4 (emitter of optocoupler transistor) is at the same potential as the ground / negative terminal/point of BAT1 – the separate/isolated power supply. So Q3 turns on and pulls Q1 gate low. Thus MOSFET Q1 is driven off.

Note that the optocoupler ground is the same ground as the ground of the MOSFET-based circuit.

Here’s the current flow for when the “Drive Signal” is a logic high.

 Fig. 4 - Current flow when driving a high-side N-channel MOSFET on, from a separate/isolated power supply (click image to enlarge)

This driver can be used for any duty cycle – all the way from 0% to 100%. The driving frequency is limited by the speed of the optocoupler. For high frequencies, optically isolated MOSFET drivers may be used instead of the two transistors and the optocoupler – the optically isolated MOSFET driver will be all that’s needed. Some such drivers are TLP250, TLP350, HCPL3120, etc.

Now let’s talk about the boostrap based drive. Here when the high-side MOSFET is off, a capacitor is charged from the driving voltage. The capacitor charges through the load or a supporting low-side MOFSET. When the high-side MOSFET is to be turned on/driven, the voltage on the capacitor is used to drive the high-side MOSFET. Thus the limitation of this method is quite obvious. A large enough capacitor should be used for storing the required energy/charge for keeping the high-side MOSFET on for the required time. At the same time, the capacitor must be large enough that during the entire driving time, the voltage doesn't fall below about 8V, in order to prevent the MOSFET from being only partially on. Thus, the bootstrap based drive can not be used for 100% or close to 100% duty cycle. And the lower the frequency of operation, the larger the required capacitance.

The easiest way to drive a MOSFET using the boostrap based drive is to use a dedicated high side MOSFET driver. Some drivers come with just the high-side driver while many come with both high-side and low-side drivers. IR2117, for example, is one driver that contains a single driver that can be used to drive a high-side MOSFET driver. IR2110, which is arguably the most popular high-low side MOSFET driver, features a high-side driver and a low-side driver in a single device. I’ve written a detailed tutorial regarding the use of IR2110. Here's the tutorial:

http://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

Besides the IR2110, there are many high-low side drivers available. L6385E and NCP2181 are my personal favorites. You can choose from a wide range of high side and high-low side drivers.

No matter which method you choose, once you know how to handle the drive requirement, it's really easy. In most cases, I use the bootstrap based drivers, although I do occasionally use isolated power-supply based drive. None of these methods are too difficult and I hope I've managed to provide you a clear answer to your question: "Why are high-side N-channel MOSFETs to be driven differently from low-side N-channel MOSFETs, and how do we drive the high-side N-channel MOSFET?" Do let me know your comments and feedback.