AN10881
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Application note
Rev. 2 — 26 September 2011
26 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
5.5.4.3
Lower voltage on SUPHS
During normal operation, each time the Half-Bridge (HB) node is switched to ground level,
the bootstrap function charges the SUPHS capacitor. Because of the voltage drop across
the bootstrap diode, the value of SUPHS is normally lower than SUPREG (or other
bootstrap supply input).
The voltage drop across the bootstrap diode is directly related to the amount of current
that is required to charge SUPHS. The resultant SUPHS voltage also has a relation to the
time available for charging.
A large voltage drop occurs when an external MOSFET with a large gate capacitance has
to be switched at high frequency (high current and a short time).
Also, during Burst mode operation, a low voltage on SUPHS can occur. In Burst mode
there are (long) periods of not switching and therefore no charging of SUPHS. During this
time the circuit supplied by SUPHS slowly discharges the supply voltage capacitor. When
a new burst starts, the SUPHS voltage is lower than during normal operation. During the
first switching cycles SUPHS is recharged to its normal level. During Burst mode, at low
output power, the switching frequency is normally rather high which limits a fast recovery
of the SUPHS voltage.
Although in most applications the voltage drop is limited, it is an important issue to be
evaluated. It can influence the selection of the best diode type for the bootstrap function
and the value of the buffer capacitor on SUPHS.
5.5.5 SUPREG power consumed by MOSFET drivers
During operation the drivers GATEPFC, GATELS and GATEHS charging the gate
capacitances of the external MOSFETs are a major part of the power consumption from
SUPREG. The amount of energy required in time is linear to the switching frequency.
Often, for the MOSFETs used, the total charge is specified for certain conditions. With this
figure an estimation can be made for the amount of current needed from SUPREG.
5.5.5.1
GATELS and GATEHS (driving a total of two MOSFETs)
(1)
Fig 12. Typical application of SUPHS
001aal024
14 SUPHS
GATEHS
GATELS
15 HB
9 SUPREG
V
BOOST
TEA1713
I
SUPIC
2
Q
gate
f
bridge
=
