3706M-E1 データシートの表示（PDF） - BCD Semiconductor

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3706M-E1

PRIMARY SIDE CONTROL IC FOR OFF-LINE BATTERY CHARGERS

BCD Semiconductor 

Data Sheet

PRIMARY SIDE CONTROL IC FOR OFF-LINE BATTERY CHARGERS AP3706

Operation Description (Continued)

Where the Vd is the diode forward drop voltage.

See equation 5

The relationship between the output constant-current

and secondary peak current Ipks is given by:

Iout = 1 × Ipks× Tons ................(7)

2

Tons+ Toffs

0V

2/3 Tons

Tons

Figure 12. Auxiliary Voltage Waveform

The output voltage is different from the secondary

voltage in a diode forward drop voltage. The diode

drop voltage 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 at two-thirds of the D1 on-time. The

CV loop control function of AP3706 then generates a

D1 off-time to regulate the output voltage.

Constant Current Operation

The AP3706 is designed to work in constant-current

(CC) mode. Figure 13 shows the secondary current

waveforms.

At the instant of D1 turn-on, the primary current

transfers to the secondary at an amplitude of:

Ipks = NP × Ipk

NS

................(8)

Thus the output constant-current is given by:

Iout = 1 × NP × Ipk× Tons = 2 × NP × Ipk

2 NS

Tons+ Toffs 7 NS

................(9)

Leading Edge Blanking

When the power switch is turned on, a turn-on spike

will occur on the sense-resistor. To avoid false-

termination of the switching pulse, a 430ns leading-

edge blanking is built in. During this blanking period,

the current sense comparator is disabled and the gate

driver can not be switched off.

See equation 8

CCM Protection

The AP3706 is designed to operate in discontinuous

conduction mode (DCM) in both CV and CC modes.

Is

Iout

To avoid operating in continuous conduction mode

0A

(CCM), the AP3706 detects the falling edge of the FB

input voltage on each cycle. If a 0.1V falling edge of

Tons

Toffs

FB is not detected, the AP3706 will stop switching.

Figure 13. Secondary Current Waveform

In CC operation, the CC loop control function of

AP3706 will keep a fixed proportion between D1 on-

time Tons and D1 off-time Toffs by discharging or

charging the capacitance connected in COMP pin. The

fixed proportion is

Tons = 4

Toffs 3

................(6)

Jul. 2008 Rev. 1.3

OVP & OCkP

The AP3706 includes output over-voltage protection

(OVP) and open circuit protection (OCkP) circuitry as

shown in Figure 14. If the voltage at FB pin exceeds

8V, 100% above the normal detection voltage, or the

-0.7V falling edge of the FB input can not be

monitored, the AP3706 will immediately shut off and

enters hiccup mode. The AP3706 sends out a fault

detection pulse every 8ms in hiccup mode until the

fault has been removed.

BCD Semiconductor Manufacturing Limited

9

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