High-side gate drivers are used to drive a MOSFET or IGBT that is connected to the positive supply and is not ground-referenced but is floating. High-side drivers are more complicated than low-side drivers because of the required voltage translation to the supply and because it is more difficult to turn off a floating transistor.

Gate drivers are available as ICs, such as the IR2112, that drives gates elevated to several hundred volts. A high and low-side driver are provided for about $1.50 in a 14-pin IC. Other suppliers, such as Intersil and Siliconix, also have integrated high-side drivers. These are typically implemented by floating an RS flop off the high-side supply rail and using two ground-referenced (low-side) high-voltage transistors to pulse the R and S inputs. The RS-flop output is buffered by a high-current driver, with bootstrapping circuitry to provide gate-voltage drive (V_GH) above the supply rail (V_g). In addition to the basic driver function, short-circuit, undervoltage, and thermal protection functions are sometimes added. Integrated gate drivers are a good alternative in many applications, but if space is not limited, a discrete solution using commodity transistors, diodes, resistors and a capacitor or two can be lower in cost. Consequently, a survey of direct-coupled high-side drivers follows.

The first direct-coupled circuit is shown below. A logic high level at the input turns MOSFET Q5 off. When Q8 conducts, it pulls its collector voltage to near-zero volts, and the gate capacitance is discharged through D4 and Q8. For motor-drive applications, the induced voltage of the motor can drive the output, causing current to flow through the gate protection Zener, D3, diode D4, and Q8 without current limiting. R5 is added to limit reverse current. If R5 must be made so large that gate drive switching times become too slow, then speed-up capacitor C2 is added. Because Q8 saturates when turned on, C1 speeds up transition times. This particular circuit turns off slowly, but quicker turn-off is easily achieved by reducing the value of R8 at the expense of greater power dissipation when Q8 is on.

This driver has the disadvantage that Q8 must pull down to ground, causing its collector to swing the full supply voltage. This large dv/dt causes electrical noise and large parasitic capacitive currents. But it is a discrete minimum-part circuit that can be optimal for drivers operating under 100 V. The parts cost is less that $0.50 (omitting Q5).

Another basic driver is shown below. It mainly differs from the previous driver in that it uses an inverting PNP output stage to drive a floating gate. When Q1 turns on, it drives Q2 on, which drives the power MOSFET (Q4) gate. Gate turn-off is accomplished by the floating PNP, Q3, which is driven on (once Q2 is off) by the voltage across the gate-source capacitance, through R4. Diode D1 is off during turn-off. This circuit has an advantage over the inverting driver in that the output circuit driving the gate is floating, isolated by the collector of Q2. Consequently, for motor-drives, no reverse current path from output through Zener D2 exists. At power-on, a conduction path through the collector of Q2 can be thwarted if problematic by placing a diode in series with the Q2 collector.

This particular circuit, when using a supply of 20 V, has a turn-on time of about 1 ms, a turn-off delay of about 2 ms and a turn-off time of about 2 ms, a low-power but excessively slow circuit. A much-improved circuit is shown below.

The biggest improvement is to remove R2, C3 and add R10. The collector current is now determined by R10 and the logic high level applied at the input, to the base of Q11. While on, power that would have been dissipated in R10 transfers to Q11, and for high-voltage applications, R2 and C3 may be included, but at an R2 value that will not saturate Q11. This change decreases switching time significantly.

To decrease turn-off time, R11 is reduced in value to discharge the base of Q10 and turn it off quicker. A smaller R11 does not appreciably increase power dissipation when Q11 is on. Similarly, by decreasing R7, Q9 base current is increased during turn-off, causing Q9 to conduct more current out of the gate, turning it off quicker. A smaller value of R7 will also require greater on-time drive current from Q10, of about V_G(on)/R7. But this is a small fraction of the gate charging current, and R7 can afford to be made relatively small. The improved non-inverting driver has a gate turn-on time of about 400 ns and a comparable turn-off time.

The additional transistor over the inverting driver is more than cost-compensated by elimination of two capacitors and a resistor. Both schemes, with low-side driver (2 BJTs, 2 Rs, 1 D, 1 C) included, cost about $0.40 (1000s), cheap enough to provide a three-phase full-bridge motor driver for the cost of a single half-bridge IC. What compensates somewhat in favor of the IC is the additional board and parts placement cost in assembly. And the added protection functions (which also cost little to implement with discrete parts) must also be taken into account. If you have the space for a discrete solution, it can sometimes be optimal, especially with supplies under 100 V.

The two basic discrete, direct-coupled, high-side drivers are sometimes a preferred alternative to an IC driver, and are cheaper and occupy less board area than transformer-coupled drivers, mainly because of the transformer size and cost. Opto-coupler circuits can also be low in parts count but are somewhat more expensive due to the coupler. While transformers and couplers offer isolation, an isolated (floating)V_GH supply may also be required, to swing along with the gate voltage.

Ó Dennis L. Feucht, 2001