MP6973

– FAST TURN-OFF INTELLIGENT RECTIFIER

MP6973 Rev. 1.0

www.MonolithicPower.com

11

6/17/2020

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APPLICATION INFORMATION

Slew Rate Detection

During

DCM

operation,

the

demagnetizing

ringing may bring VDS below 0V. If VDS reaches

the turn-on threshold during the ringing period,

SR controllers without slew rate detection may

turn on the MOSFET by mistake. This not only

increases power loss, but may also lead to

shoot-through if the primary-side MOSFET is

turned on within the minimum on time of the SR

controller.

The falling slew rate of the ringing is always

much less than when the primary MOSFET is

turned off; this false turn-on situation can be

prevented by the slew rate detection function.

When the slew rate is less than the threshold, the

IC does not turn on the gate even when VDS

reaches the turn-on threshold. For more details,

see the Turn-On Phase section on page 10.

External Resistor on SENSE and HVC

Over-voltage conditions may lead to the device

malfunctioning or even being damaged, so the

application design must be careful to guarantee

safe operation, especially on the high-voltage

pin.

One common over-voltage condition occurs

when the body diode of the SR MOSFET is

turned on, as the forward voltage drop may

exceed the negative rating on the SENSE pin. In

this case, it is recommended to place an external

resistor between SENSE and the MOSFET

drain. The resistance is typically recommended

to be between 100

Ω and 300Ω.

On the other hand, this resistor also cannot be

too large, because it may slow down the slew

rate on VDS detection. In general, it is not

recommended to use a resistor greater than

300

Ω, but this should be checked for each case

based on the condition of the slew rate.

In the applications where HVC may also suffer

from negative voltage bias (e.g. in the high side

setup without auxiliary winding), there should be

also the same resistance be placed on HVC

externally.

Typical System Implementations

Figure

3

shows

the

typical

system

IC

implementation in low-side rectification. The

MP6973 is directly supplied by the output.

MP6973

SENSE

VS

VDD

VD

HVC

VOUT+

VOUT-

Figure 3: Low-Side Rectification

Maximum Output Current

The allowed temperature rise of the MP6973

limits the maximum output current the device can

handle. The temperature rise is determined by its

own power loss. Generally, the MP6973

’s

recommended

rated

output

current

for

a

universal input adapter is 3A. For certain designs,

the power loss of the MP6973 can be calculated,

so the maximum output current can be deduced.

The MP6973

’s power loss can be separated into

several parts, including controller consumption

and integrated MOSFET conduction loss. If the

MP6973 works in continuous conduction mode

(CCM), reverse-recovery loss of the integrated

MOSFET must also be considered. Each part of

the loss can be calculated based on Equation (1),

Equation (2), and Equation (3), respectively:



LOSS_CONTROLLER

HVC

DD

P

V

I

(1)

S_ON

t

LOSS _ SR _ CONDUCTION

SW

SR _ SD

SR _ SD

0

P

f

V

(t) I

(t)dt









(2)

 







LOSS _ SR _ RR

DS

RR

RR

SW

1

P

V

I

t

f

2

(3)

Where IDD is the current of the MP6973, VHVC is

the voltage on HVC pin, tS_ON is the SR on period,

VSR_SD is the voltage drop from the SR, ISR_SD is

the current flowing through the SR, IRR is the

peak reverse current, and tRR is the reverse-

recovery time.

The total loss of the MP6973 (PLOSS) is the sum

of the above losses. If an RC snubber is used,

the power loss caused by this snubber must also

be taken into consideration.

The junction and case temperature rises can be

calculated with the junction-to-ambient thermal

resistance (

θJA) and junction-to-case thermal

resistance (

θJC). The junction temperature must

be within ABS (typically 150°C). Calculate

∆TJA

and

∆TJC with Equation (4) and Equation (5):