Philips L01.1L AC, Service Manual

Circuit Description

EN 76

L01.1L AC

9.

Figure 9-7 QR-mode time intervals

In the Quasi-Resonant mode each period can be divided into 
four different time intervals, in chronological order:
•

Interval 1: t0 < t < t1 primary stroke. At the beginning of 
the first interval, the MOSFET is switched 'on' and energy 
is stored in the primary inductance (magnetization). At the 
end, the MOSFET is switched 'off' and the second interval 
starts.

•

Interval 2: t1 < t < t2 commutation time. In the second 
interval, the drain voltage will rise from almost zero to 
V_in+n•(V_out+V_f). V_f is the forward voltage drop of de 
diode that will be omitted from the equations from now on. 
The current will change its positive derivative, 
corresponding to V_in/L_p, to a negative derivative, 
corresponding to -n•(V_out/L_p).

•

Interval 3: t2 < t < t3 secondary stroke. In the third 
interval, the stored energy is transferred to the output, so 
the diode starts to conduct and the inductive current Il will 
decrease. In other words, the transformer will be 
demagnetized. When the inductive current has become 
zero the next interval begins.

•

Interval 4: t3 < t < t00 resonance time. In the fourth 
interval, the energy stored in the drain capacitor C_d will 
start to resonate with the inductance L_p. The voltage and 
current waveforms are sinusoidal waveforms. The drain 
voltage will drop from V_in+(n•V_out) to V_in-(n•V_out).

Frequency Behavior
The frequency in the QR-mode is determined by the power 
stage and is not influenced by the controller (important 
parameters are L_p and C_d). The frequency varies with the 
input voltage V_in and the output power P_out. If the required 
output power increases, more energy has to be stored in the 
transformer. This leads to longer magnetizing t_prim and 
demagnetizing t_sec times, which will decrease the frequency. 
See the frequency versus output power characteristics below. 
The frequency characteristic is not only output power-, but also 
input voltage dependent. The higher the input voltage, the 
smaller t_prim, so the higher the frequency will be.

Figure 9-8 QR frequency behavior

Point P1 is the minimum frequency f_min that occurs at the 
specified minimum input voltage and maximum output power 
required by the application. Of course, the minimum frequency 
has to be chosen above the audible limit (>20 kHz). 

Start-Up Sequence
When the rectified AC voltage V_in (via the center tap 
connected to pin 8) reaches the Mains dependent operation 
level (Mlevel: between 60 and 100 V), the internal 'Mlevel 
switch' will be opened and the start-up current source is 
enabled to charge capacitor C2521 at the V_cc pin as shown 
below. 
The 'soft start' switch is closed when the V_cc reaches a level 
of 7 V and the 'soft start' capacitor C_ss  (C2522, between pin 
5 and the sense resistor R3526), is charged to 0.5 V. 
Once the V_cc capacitor is charged to the start-up voltage 
V_start  (11 V), the IC starts driving the MOSFET. Both internal 
current sources are switched 'off' after reaching this start-up 
voltage. Resistor R_ss (3524) will discharge the 'soft start' 
capacitor, such that the peak current will slowly increase. This 
to prevent 'transformer rattle'. 
During start-up, the V_cc capacitor will be discharged until the 
moment that the primary auxiliary winding takes over this 
voltage.

Figure 9-9 Start-up behavior

V

IN

V

GATE

V

D

n

⋅

V

OUT

I

L

C

D

C

OUT

D

L

P

C

IN

0

Demagneti-

zation

V

GATE

V

D

I

L

0

Magnetization

t

1

t

2

t

3

t

00

T

t

0

1

2

3

4

Valley

n

⋅

V

OUT

V

IN

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V

IN_MAX

P

OUT_MIN

                          P

OUT_MAX

power

switching
frequency

f

MIN

f

MAX

V

IN_MIN

P1

P2

   QR frequency characteristics at different input voltages

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0.5V

+

-

I

L

R

SENSE

I

SS

V

OCP

R

SS

C

SS

soft start

V

IN

8

5

1

Mlevel

I

in(Vcc)

V

SENSE

2

C

Vcc

V

CC

Charging of V

CC

 capacitor

taken over by the winding

V

(start)

=11V

C

Vcc

 charged

 by current

V

OUT

V

GATE

V

CC

V

SENSE

I

L

τ

 = R

SS

⋅

C

SS

≈

7V

 Start-up sequence

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