MIC5016/5017

Micrel

5-154

October 1998

High Side Driver With Load Protection (Figure 12) Al-

though the MIC5016/17 devices are reverse battery pro-

tected, the load and power FET are not in a typical high side

configuration. In the event of a reverse battery condition, the

internal body diode of the power FET will be forward biased.

This allows the reversed supply to drive the load.

An MBR2035CT dual Schottky diode was used to eliminate

this problem. This particular diode can handle 20A continu-

ous current and 150A peak current; therefore it should survive

the rigors of an automotive environment. The diodes are

paralleled to reduce the switch loss (forward voltage drop).

Figure 12: High Side Driver WIth Load Protection

Push-Pull Driver With No Cross-Conduction (Figure 13)

As the turn-off time of the MIC5016/17 devices is much faster

than the turn-on time, a simple dual push-pull driver with no

cross conduction can be made using one MIC5016 and one

MIC5017. The same control signal is applied to both inputs;

the MIC5016 turns on with the positive signal, and the

MIC5017 turns on when it swings low.

Figure 13: Push-Pull Driver

This scheme works with no additional components as the

relative time difference between the rise and fall times of the

MIC5014 is large. However, this does mean that there is

considerable deadtime (time when neither driver is turned

on). If this circuit is used to drive an inductive load, catch

diodes must be used on each half to provide an alternate path

for the kickback current that will flow during this deadtime.

This circuit is also a simple H-bridge which can be driven with

a PWM signal on the input for SMPS or motor drive applica-

tions in which high switching frequencies are not desired.

Synchronous Rectifier (Figure 14) In applications where

efficiency in terms of low forward voltage drops and low diode

reverse-recovery losses is critical, power FETs are used to

achieve rectification instead of a conventional diode bridge.

Here, the power FETs are used in the third quadrant of the IV

characteristic curve (FETs are installed essentially “back-

wards”). The two FETs are connected such that the top FET

turns on with the positive going AC cycle, and turns off when

it swings negative. The bottom FET operates opposite to the

top FET.

In the first quadrant of operation, the limitation of the device

is determined by breakdown voltage. Here, we are limited by

the turn-on of a parasitic p-n body drain diode. If it is allowed

to conduct, its reverse recovery time will crowbar the other

power FET and possibly destroy it. The way to prevent this

is to keep the IR drop across the device below the cut-in

voltage of this diode; this is accomplished here by using a fast

comparator to sense this voltage and feed the appropriate

signal to the control inputs of the MIC5016 device. Obviously,

it is very important to use a comparator with a fast slew rate

such as the LM393, and fast recovery diodes. 3mV of positive

feedback is used on the comparator to prevent oscillations.

At 3A, with an R

per FET is ~ 0.2 V as opposed to the 0.7 to 0.8 V drop that a

normal diode would have. Even greater savings can be had

by using FETs with lower R

that the peak currents and voltages do not exceed the SOA

of the chosen FET.

Figure 14: High Efficiency 60 Hz

Synchronous Rectifier

1/2 MIC5016

Control Input

OFF

ON

IRF540

12V

NC

Gate

Gnd

Source

Input

V+

NC

NC

10µF

MBR2035CT

MIC5016

Control Input 1

12

10

3

11

IRFZ40

12V

Gate A

Source B

Gnd

In B

In A

V+ A

Source A

Gate B

10µF

MIC5017

12

10

14

11

4

2

6

5

Gate A

Gate B

In B

In A

V+ B

V+ A

Source A

Source B

12V

IRFZ40

Gnd

3

V+ B

Control Input 2

14

IRFZ40

IRFZ40

4

2

6

5

MIC5016

12

10

11

3

4

2

6

5

Gate A

Gate B

Gnd

In B

In A

V+ A

Source A

Source B

10µF

V+ B

14

56k

Ω

10

Ω

110V AC

Caltronics

T126C3

25.2V

30m

Ω

10k

Ω

10k

Ω

1/2 LM393

1N914 (2)

1k

Ω

1k

Ω

*

1N914

1RF540

4700µF

18V, 3A

*

1RF540

1N914

* Parasitic body diode