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TK75050

SMART MOSFET DRIVER

Toko America Inc  

TK75050

APPLICATION INFORMATION (CONT.)

ADDITIONAL OVERLOAD PROTECTION IN PEAK-

CURRENT-CONTROLLED CONVERTERS

Figure 5 shows the TK75050 in a current-mode-controlled

forward converter, with optically isolated feedback. In this

application the TK75050 helps to achieve a tightly controlled

current-limit-characteristic. A tight current limit cannot

usually be achieved with only current-mode control due to

the presence of the stabilizing ramp. The lack of the

stabilizing ensures that the knee current (i.e., the output

current where the limiting begins) is only slightly lower than

the short-circuit current. The difference between the knee

current and the short-circuit current is about one-half of the

ripple current in the filter inductor. If a stabilizing ramp were

added to the current-sense signal, the difference would be

significantly higher.

VIN

+

+12 V

OC

TK75050

CURRENT-MODE

PWM

CONTROLLER

VIN

+12 V

TK75050

-

+

+

-

+

FEEDBACK

FIGURE 6: TK75050 IN A CONVERTER WITH

AVERAGE CURRENT CONTROL

FLOATING DRIVE WITH OVERLOAD PROTECTION

The TK75050 can be used as a driver and current limiter

for a floating power switch. Figure 7 shows the IC in a buck

converter with transformer-isolated drive.

VIN

FIGURE 5: TK75050 IN A PEAK-CURRENT-

CONTROLLED CONVERTER

ADDITIONAL OVERLOAD PROTECTION IN AVERAGE-

CURRENT-CONTROLLED CONVERTERS

+

TK75050

In converters with average current control the peak current

information is lost and the response of the current-control

loop slows down. The speed of the current-control loop

may not be sufficient to provide effective protection against

sudden overload or saturation of an inductor or transformer.

Figure 6 shows an average-current-controlled boost

converter where the TK75050 provides additional fast

overload protection.

FIGURE 7: TK75050 AS A FLOATING DRIVER IN A

BUCK CONVERTER

DEMO BOARD

The purpose of the board is to demonstrate the high-speed

current-limit capability of Toko's TK75050 smart MOSFET

driver. In the board a 2-A/500 V MOSFET switch is turned

on directly (i.e., without any series impedance) into a DC-

bus with up to 400 V, at a frequency of 30 kHz. By removing

the short across a 3.3 µH inductor in series with the

MOSFET, it is also possible to investigate the effect of the

wiring inductance between the switch and the load. In

January 1999 TOKO, Inc.

Page 9

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