AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
60 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
enabled. When both the positive and negative OCR levels are exceeded, the soft-start
discharge current flows continuously. In this way the operating frequency is slowly
increased until the resonant current value just reaches the value permitted by the preset.
The behavior during OCR can be observed on the SSHBC/EN pin as a resultant
regulation voltage.
When an OCR situation is present for a long time, a serious fault condition is assumed.
During OCR the protection timer is activated. The charging of the protection timer is active
approximately a half period cycle after the
0.5 V level is exceeded. If the detection levels
are continuously exceeded, the timer is charged continuously but, if the detection levels
are only sometimes exceeded, the timer is charged accordingly (for details on
charging/discharging of the protection timer refer to
). The restart state is
activated when RCPROT reaches the protection level of 4 V.
8.7.1.1
Start-up
The overcurrent regulation is 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 can 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. However, the OCR can not be fast enough to limit the current for certain
error conditions. OverCurrent Protection (OCP) is implemented to protect against those
error conditions.
The internal OCP level is set at
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 selected to sufficiently limit the
output power under these conditions.
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 reactivated.
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 overcurrent protection and simplifies the stability of
the overcurrent regulation. The OCR and OCP comparators compare the SNSCURHBC
voltage to the maximum positive and negative values.
The primary current is higher for the same output power when the boost voltage is low. A
boost compensation is included to reduce the dependency of the protected output current
level for the boost voltage. 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
.
