L6598D データシートの表示（PDF） - STMicroelectronics

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L6598D

HIGH VOLTAGE RESONANT CONTROLLER

STMicroelectronics 

L6598

3.3 Bootstrap Section

The supply of the high voltage section is obtained by means of a bootstrap circuitry. This solution normally re-

quires an high voltage fast recovery diode for charging the bootstrap capacitor (fig. 14a). In the device a patent-

ed integrated structure, replaces this external diode. It is realised by means of a high voltage DMOS, driven

synchronously with the low side driver (LVG), with in series a diode, as shown in fig. 14b.

Figure 14. Bootstrap driver

DBOOT

VS

VS

VBOOT

VBOOT

VOUT

CBOOT

LVG

VOUT

CBOOT

a

b

To drive the synchronised DMOS it is necessary a voltage higher than the supply voltage Vs. This voltage is

obtained by means of an internal charge pump (fig. 14b).

The diode connected in series to the DMOS has been added to avoid undesirable turn on of it. The introduction

of the diode prevents any current can flow from the Vboot pin to the VS one in case that the supply is quickly

turned off when the internal capacitor of the pump is not fully discharged.

The bootstrap driver introduces a voltage drop during the recharging of the capacitor Cboot (i.e. when the low

side driver is on), which increases with the frequency and with the size of the external power MOS. It is the sum

of the drop across the RDSON and of the diode threshold voltage. At low frequency this drop is very small and

can be neglected. Anyway increasing the frequency it must be taken in to account. In fact the drop, reducing

the amplitude of the driving signal, can significantly increase the RDSON of the external power MOS (and so the

dissipation).

To be considered that in resonant power supplies the current which flows in the power MOS decreases increas-

ing the switching frequency and generally the increases of RDSON is not a problem because power dissipation

is negligible. The following equation is useful to compute the drop on the bootstrap driver:

Vdrop

=

Ich argeRdson + Vdiode → Vdrop

=

-------Q-----g-------

Tch arge

R

ds

o

n

+

Vdiode

[8]

where Qg is the gate charge of the external power MOS, Rdson is the on resistance of the bootstrap DMOS, and

Tcharge is the time in which the bootstrap driver remains on (about the semiperiod of the switching frequency

minus the dead time). The typical resistance value of the bootstrap DMOS is 150 Ohm. For example using a

power MOS with a total gate charge of 30nC the drop on the bootstrap driver is about 3V, at a switching fre-

quency of 200kHz. In fact:

Vdrop = 2--3--.--20---3n----µC---s--150Ω + 0.6V~2.6V

To summarise, if a significant drop on the bootstrap driver (at high switching frequency when large power MOS

are used) represents a problem, an external diode can be used, avoiding the drop on the RDSON of the DMOS.

10/17

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