Philips Semiconductors
Product specification
STARplugTM
TEA152x family
FUNCTIONAL DESCRIPTION
Duty factor control
The TEA152x family is the heart of a compact flyback
converter, with the IC placed at the primary side. The
auxiliary winding of the transformer can be used for
indirect feedback to control the isolated output. This
additional winding also powers the IC. A more accurate
control of the output voltage and/or current can be
implemented with an additional secondary sensing circuit
and optocoupler feedback.
The duty factor is controlled by the internal regulation
voltage and the oscillator signal on pin RC. The internal
regulation voltage is equal to the external regulation
voltage (minus 2.5 V) multiplied by the gain of the error
amplifier (typical 20 dB (10 ×)).
Valley switching (not implemented in TEA152xAJM
versions)
The TEA152x family uses voltage mode control. The
frequency is determined by the maximum transformer
demagnetizing time and the time of the oscillator. In the
first case, the converter operates in the Self Oscillating
Power Supply (SOPS) mode. In the latter case, it operates
at a constant frequency, which can be adjusted with
external components RRC and CRC. This mode is called
Pulse Width Modulation (PWM). Furthermore, a primary
stroke is started only in a valley of the secondary ringing.
This valley switching principle minimizes capacitive
switch-on losses.
A new cycle is started when the primary switch is switched
on (see Fig.5). After a certain time (determined by the
oscillator voltage RC and the internal regulation level), the
switch is turned off and the secondary stroke starts. The
internal regulation level is determined by the voltage on
pin REG. After the secondary stroke, the drain voltage
shows an oscillation with a frequency of approximately
1
---------------------------------------------------
(2 × π × (Lp × Cp))
where Lp is the primary self inductance and Cp is the
parasitic capacitance on the drain node.
Start-up and under voltage lock-out
As soon as the oscillator voltage is high again and the
secondary stroke has ended, the circuit waits for a low
drain voltage before starting a new primary stroke.
Figure 5 shows the drain voltage together with the valley
signal, the signal indicating the secondary stroke and the
RC voltage.
Initially, the IC is self supplying from the rectified mains
voltage. The IC starts switching as soon as the voltage on
pin VCC passes the VCC(start) level. The supply is taken
over by the auxiliary winding of the transformer as soon as
VCC is high enough and the supply from the line is stopped
for high efficiency operation.
The primary stroke starts some time before the actual
valley at low ringing frequencies, and some time after the
actual valley at high ringing frequencies. Figure 6 shows a
typical curve for a reflected output voltage N × Vo of 80 V.
This voltage is the output voltage Vo (see Fig.7)
transferred to the primary side of the transformer with the
factor N (determined by the turns ratio of the transformer).
Figure 6 shows that the system switches exactly at
minimum drain voltage for ringing frequencies of 480 kHz,
thus reducing the switch-on losses to a minimum.
At 200 kHz, the next primary stroke is started at 33° before
the valley. The switch-on losses are still reduced
significantly.
When for some reason the auxiliary supply is not sufficient,
the high voltage supply also supplies the IC. As soon as
the voltage on pin VCC drops below the VCC(stop) level, the
IC stops switching and restarts from the rectified mains
voltage.
Oscillator
The frequency of the oscillator is set by the external
resistor and capacitor on pin RC. The external capacitor is
charged rapidly to the VRC(max) level and, starting from a
new primary stroke, it discharges to the VRC(min) level.
Because the discharge is exponential, the relative
sensitivity of the duty factor to the regulation voltage at low
duty factor is almost equal to the sensitivity at high duty
factors. This results in a more constant gain over the duty
factor range compared to PWM systems with a linear
sawtooth oscillator. Stable operation at low duty factors is
easily realized. For high efficiency, the frequency is
reduced as soon as the duty factor drops below a certain
value. This is accomplished by increasing the oscillator
charge time.
Demagnetization
The system operates in discontinuous conduction mode all
the time. As long as the secondary stroke has not ended,
the oscillator will not start a new primary stroke. During the
first tsuppr seconds, demagnetization recognition is
suppressed. This suppression may be necessary in
applications where the transformer has a large leakage
inductance and at low output voltages.
2000 Sep 08
6
NXP [ NXP ]
