AN1537 APPLICATION NOTE

10/18

1

The oscillator of the L5991 has been modified to maintain the same frequency under normal opera-

tion (65kHz) at full load and have a standby frequency of 36kHz. Also in this case the oscillator fre-

quency with no-load is chosen 5kHz. Further details on the calculations can be found in the appendix.

2

Again, the dummy load represented by the feedback components on the secondary side has been

reduced at about 40mW (see the same point of the previous design).

3

The frequency compensation of the voltage control loop (C7, C14, R20) has been modified so as to

double the bandwidth and get a faster response (especially useful at start-up).

4

R28 has been decreased from 3.3 to 1

Ω, to prevent the supply voltage of the L5991A from going below

the UVLO threshold with no load, while not exceeding the rating of both the L5991A and the L6561.

5

The multiplier bias resistors (R3A = R3B = 680k

Ω and R1 = 10 kΩ) of the L6561 consume 103mW

@264 Vac, then their value has been more than doubled (R3A = R3B = 1.5M

Ω and R1 = 22kΩ). In

this way their maximum consumption is reduced at 46mW @264 Vac.

6

The output divider of the PFC pre-regulator (R7A = R7B = 499k

Ω and R1 = 6.34kΩ) gives origin to

a consumption of 140mW @264 Vac. With the aim of reducing the consumption at 40mW maximum,

while not degrading the dynamic OVP function, the feedback network has been modified as shown

in figure 13. The capacitor bypasses the two 1.5M

Ω resistors during output voltage overshoots, re-

sulting in a only slightly higher OVP threshold, as compared to the original one. The positive side-

effect is an improvement of the transient response of the PFC pre-regulator. The components for the

frequency compensation of the error amplifier of the L6561 have been changed consequently.

The modifications are summarized in table 5.

Figure 13. Modified PFC pre-regulator's feedback and frequency compensation network

Table 5. 80W AC-DC adapter with PFC: list of modifications to the original design

Part

Original value

New Value

Part

Original value

New Value

R1

10k

Ω

22k

Ω

R31

2.2k

Ω

20k

Ω

R3A, R3B

680k

Ω

1.5M

Ω

R33

13k

Ω

33k

Ω

R6A

6.34k

Ω

22k

Ω

R34

510k

Ω

---

R7A, R7B

499k

Ω

1.5M

Ω

R35

2.2k

Ω

shorted

R7C

---

499k

Ω

R36

10k

Ω

5.6k

Ω

R15

10k

Ω

15k

Ω

---

7.5k

Ω

R16

22k

Ω

11k

Ω

C13

15nF

3.3nF

R28

3.3

Ω

1

Ω

C25

330nF

3.3nF

R29

348

Ω

3.16k

Ω

C30

---

33nF / 400V

R30

1.2k

Ω

---

D13, D14

---

1N4148

R7A

1.5M

R6A

22K

2

L6561

PFC out

C8 1

µF

R7C

499K

C30

33 nF

400V

R7B

1.5M