High Sidegated River

High-Side Gate Drivers

High-side gate drivers are used to drive a MOSFET or IGBT that is connected to thepositive supply and is not ground-referenced but is floating. High-side drivers aremore complicated than low-side drivers because of the required voltage translation tothe 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 toseveral hundred volts. A high and low-side driver are provided for about $1.50 in a14-pin IC. Other suppliers, such as Intersil and Siliconix, also have integratedhigh-side drivers. These are typically implemented by floating an RS flop off thehigh-side supply rail and using two ground-referenced (low-side) high-voltagetransistors to pulse the R and S inputs. The RS-flop output is buffered by ahigh-current driver, with bootstrapping circuitry to provide gate-voltage drive (VGH)

above the supply rail (Vg). In addition to the basic driver function, short-circuit,

undervoltage, and thermal protection functions are sometimes added. Integrated gatedrivers are a good alternative in many applications, but if space is not limited, adiscrete solution using commodity transistors, diodes, resistors and a capacitor or twocan be lower in cost. Consequently, a survey of direct-coupled high-side driversfollows.

Inverting Direct-Coupled High-Side Driver

The first direct-coupled circuit is shown below. A logic high level at the input turnsMOSFET 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

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applications, the induced voltage of the motor can drive the output, causing currentto flow through the gate protection Zener, D3, diode D4, and Q8 without currentlimiting. R5 is added to limit reverse current. If R5 must be made so large that gatedrive switching times become too slow, then speed-up capacitor C2 is added. BecauseQ8 saturates when turned on, C1 speeds up transition times. This particular circuitturns off slowly, but quicker turn-off is easily achieved by reducing the value of R8 atthe expense of greater power dissipation when Q8 is on.

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

Noninverting Direct-Coupled High-Side Driver

Another basic driver is shown below. It mainly differs from the previous driver in thatit uses an inverting PNP output stage to drive a floating gate. When Q1 turns on, itdrives Q2 on, which drives the power MOSFET (Q4) gate. Gate turn-off isaccomplished by the floating PNP, Q3, which is driven on (once Q2 is off) by thevoltage across the gate-source capacitance, through R4. Diode D1 is off duringturn-off. This circuit has an advantage over the inverting driver in that the outputcircuit driving the gate is floating, isolated by the collector of Q2. Consequently, formotor-drives, no reverse current path from output through Zener D2 exists. Atpower-on, a conduction path through the collector of Q2 can be thwarted ifproblematic 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 �s, aturn-off delay of about 2 �s and a turn-off time of about 2 �s, a low-power butexcessively slow circuit. A much-improved circuit is shown below.


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The biggest improvement is to remove R2, C3 and add R10. The collector current isnow determined by R10 and the logic high level applied at the input, to the base ofQ11. While on, power that would have been dissipated in R10 transfers to Q11, andfor high-voltage applications, R2 and C3 may be included, but at an R2 value that willnot saturate Q11. This change decreases switching time significantly.

To decrease turn-off time, R11 is reduced in value to discharge the base of Q10 andturn it off quicker. A smaller R11 does not appreciably increase power dissipationwhen Q11 is on. Similarly, by decreasing R7, Q9 base current is increased duringturn-off, causing Q9 to conduct more current out of the gate, turning it off quicker. Asmaller value of R7 will also require greater on-time drive current from Q10, of aboutVG(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-ontime of about 400 ns and a comparable turn-off time.

The additional transistor over the inverting driver is more than cost-compensated byelimination of two capacitors and a resistor. Both schemes, with low-side driver (2BJTs, 2 Rs, 1 D, 1 C) included, cost about $0.40 (1000s), cheap enough to provide athree-phase full-bridge motor driver for the cost of a single half-bridge IC. Whatcompensates somewhat in favor of the IC is the additional board and parts placementcost in assembly. And the added protection functions (which also cost little toimplement with discrete parts) must also be taken into account. If you have the spacefor a discrete solution, it can sometimes be optimal, especially with supplies under100 V.

Closure

The two basic discrete, direct-coupled, high-side drivers are sometimes a preferredalternative to an IC driver, and are cheaper and occupy less board area thantransformer-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


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due to the coupler. While transformers and couplers offer isolation, an isolated(floating)VGH supply may also be required, to swing along with the gate voltage.

� Dennis L. Feucht, 2001


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