AN10907
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Application note
Rev. 1 — 28 December 2010
21 of 82
NXP Semiconductors
AN10907
TEA1613T resonant power supply control IC
6.5.4.3
Lower voltage on SUPHS
During normal operation, each time the half-bridge node (HB) is switched to ground level,
the SUPHS capacitor is charged by the bootstrap function. 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 needed to charge SUPHS. The resultant SUPHS voltage is also influenced by 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 c 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. At the
moment a new burst starts, the SUPHS voltage is lower than during normal operation.
During the first switching cycles the 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.
6.5.5 SUPREG power consumed by MOSFET drivers
During operation the drivers GATELS and GATEHS charging the gate capacitances of the
external MOSFETs consume a major part of the power from SUPREG. The amount of
energy needed for this in time, is linear to the switching frequency. Often, for the
MOSFETs used, the total charge is specified for certain conditions. With this value an
estimation can be made for the amount of current needed from SUPREG.
GATELS and GATEHS (driving two MOSFETs in total):
I
SUPIC
= 2
Q
gate
f
bridge
For example, a MOSFET with Q
gate
= 40 nC at a bridge frequency 100 kHz:
Fig 11. Typical application of SUPHS
001aal437
SUPHS
GATEHS
GATELS
HB
SUPREG
V
BOOST
TEA1613
