HF81 – X CAPACITOR BLEEDER

HF81 Rev. 1.12

www.MonolithicPower.com

6

11/15/2012

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OPERATION

A smart bleeder normally works as an off device:

It consumes very little current, it always monitors

the AC line for zero crossings; whenever it

detects a zero crossing it dischargers the timer

capacitor and resets the latch circuit.

Whenever a prolonged period occurs without a

zero crossing (meaning the power supply unit

[PSU] was unplugged from the utility line), the

timer block times out and triggers the latch circuit.

The latch circuit turns on the discharging

transistors, as shown by the timing diagram in

Figure 1. The internal transistor enables the

bleed resistors, and stops when either the

voltage across the HF81 drops below 10V or the

PSU is plugged back into the utility line. This

operation allows for more X capacitor choices

and reduces the inductor cost.

Figure 1: Zero-Crossing Detection Timing

COMPONENT SELECTION

According to international safety requirements,

the X capacitor voltage must drop to 37% within

1s. Based on a 5% tolerance for the discharge

resistor and a 20% tolerance for the X capacitor,

select an RC time constant of 0.75s. Table 1

provides sample values for the X capacitor and

the discharge resistance.

Carefully select a metal oxide varistor (MOV) and

fuse to meet safety requirements: The MOV

suppresses voltage transients. During normal

operation, a MOV with high impedance permits

only a small current. When a voltage surge

occurs, the MOV resistance reduces to a few Ωs,

which allows the large surge current to flow

through while protecting other devices in parallel

to it.

There are several parameters for selecting a

MOV, including maximum-allowable voltage,

maximum clamping voltage, and the MOV max

endured energy. The maximum-allowable voltage

must exceed the maximum working voltage by

10%, 20% or the circuit needs an even higher

derating, which is determined by the power

source stability. After selecting a MOV, verify it

the current is 1mA. Simplified, this is:

1mA

peak

AC

V

1.5V

2.2V





(1)

is the rms working voltage.

Also, please make sure the breakdown voltage of

other components on the AC side fall below the

maximum allowable voltage.

Base MOV size selection on MOV withstanding

transient energy. Calculate the energy using

formula 2.

Clamp

Peak

WK V

I

t

 



（

2）

Where K is a constant 1.0 for rectangular

waveform and 1.4 for 8/20µs and 10/1000µs

Make sure that the calculated energy is lower

than the rated energy on the MOV datasheet.

Select the fuse based on MOV specification.

Generally, select the fuse based on MOV

diameter as per Table 2.

Zero Crossing

Detection Timing

100V/div

22ms