AN10907
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© NXP B.V. 2010. All rights reserved.
Application note
Rev. 1 — 28 December 2010
45 of 82
NXP Semiconductors
AN10907
TEA1613T resonant power supply control IC
Start-up
The OverCurrent Regulation is very effective for limiting the output current during start-up.
A smaller soft-start capacitor can be chosen which allows faster start-up. The small
soft-start capacitor may sometimes result in an excessive output current but the OCR
function can slow down the frequency sweep to keep the output current within the limits.
8.7.2 HBC overcurrent protection
In most cases the OverCurrent Regulation is able to keep the current below the set
maximum values. During certain error conditions however, the OCR might not be fast
enough to limit the current. The OverCurrent Protection (OCP) is implemented to protect
against these error conditions.
The internal OCP level is set to
1 V for SNSCURHBC. This is significantly higher than
the OCR level of
0.5 V. When the OCP level is reached the frequency immediately jumps
to the maximum via a soft-start reset procedure, followed by a normal sweep down.
The maximum frequency value for soft-start must be chosen to be able to limit the output
power in these conditions sufficiently.
The behavior during OCP can be observed on the SSHBC/EN pin as a new soft-start.
Depending on the (over)load or fault condition during this new soft-start, OCR or OCP can
be activated again.
8.7.3 SNSCURHBC boost voltage compensation
The primary current, also called resonant current, is sensed via pin SNSCURHBC. It
senses the momentary voltage across an external current sense resistor. The use of the
momentary current signal allows a fast OCP and simplifies the stability of the OCR. The
OCR and OCP comparators compare the SNSCURHBC voltage to the maximum positive
and negative values.
For the same output power, the primary current is higher when the boost voltage is low. To
reduce the dependency of the protected output current level for the boost voltage, a boost
compensation is included. The boost compensation sources and sinks a current from the
SNSCURHBC pin. This current creates a voltage drop across the series resistor Rcc. A
typical value for this resistor is 1 k
.
The amplitude of the current depends linearly on the boost voltage. At nominal boost
voltage the current is zero and the voltage across the current sense resistor is also
present at the SNSCURHBC pin. At the boost start level SNSBOOST = 1.8 V and the
current is a maximum of 170
A. The direction of the current, sink or source, depends on
the active gate signal. The voltage drop created across Rcc reduces the voltage
amplitude at the pin, resulting in a higher effective current protection level. The amount of
compensation is set by the value of Rcc.
