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AP3988

PRIMARY SIDE POWER SWITCHER FOR OFF-LINE SMPS

Diodes Incorporated. 

Operation Description (Cont.)

A Product Line of

Diodes Incorporated

AP3988/89/90

Constant Primary Peak Current

The primary ip(t) current is sensed by a current sense resistor RCS as shown in Figure 1.

The current rises up linearly at a rate of:

dip (t)  Vbulk(t) ……… (1)

dt

LM

See equation 2

IPK

ip(t)

0A

Figure 2. Primary Current Waveform

As illustrated in Figure 2, when the current ip(t) rises up to IPK, the switch Q1 turns off. The constant peak current is given by:

I PK



VCS

RCS

………(2)

The energy stored in the magnetizing inductance LM each cycle is therefore:

Eg



1

2

 LM

 I PK 2 ………(3)

So the power transferring from input to output is given by:

P



1

2

 LM

 IPK2



fSW

………(4)

Where fSW is the switching frequency. When the peak current IPK is constant, the output power depends on the switching frequency fSW.

Constant Voltage Operation

The AP3988/89/90 captures the auxiliary winding feedback voltage at FB pin and operates in constant-voltage (CV) mode to regulate the output

voltage. Assuming the secondary winding is master, the auxiliary winding is slave during the D1 on-time. The auxiliary voltage is given by:

  VAUX



N AUX

NS



VO

 Vd

………(5)

Where Vd is the diode forward drop voltage, NAUX is the turns of auxiliary winding, and NS is the turns of secondary winding.

See equation 5

VAUX

0V

Portion of Tons

Tons

Figure 3. Auxiliary Voltage Waveform

The output voltage is different from the secondary voltage in a diode forward drop voltage Vd which depends on the current. If the secondary

voltage is always detected at a constant secondary current, the difference between the output voltage and the secondary voltage will be a fixed Vd.

The voltage detection point is portion of Tons after D1 is turned on. The CV loop control function of AP3988/89/90 then generates a D1 off-time to

regulate the output voltage.

AP3988/89/90

Document number: DS36722 Rev. 3 - 2

7 of 13

www.diodes.com

March 2014

© Diodes Incorporated

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