UM10525
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1 — 19 April 2012
11 of 21
NXP Semiconductors
UM10525
120 V 20 W CFL demo board using the UBA2212
5.1 Boost lamp with high lamp voltage
During boost state, lamps consume more power. As a result, the half-bridge valley voltage
value is pulled low. In addition, CM or hard switching occurs especially when a high lamp
voltage with used with a low mains input.
The UBA2212 detects switched operation prevent stress on MOSFETs using the internal
active Zero-Voltage Switching (ZVS) control circuit.
When capacitive mode is detected, the internal current source discharges the internal
capacitor. Consequently, the SW voltage is lower and the RMS circuit controlled boost
frequency increases until the system is at the zero-voltage switching border.
Figure 6
shows the Capacitive Mode (CM) protection response in the boost state.
The ratio of boost lamp current to RMS lamp current (I
bst
: I
RMS
) is lower because of CMP.
If a significant boost effect is needed, the system parameters need adjusted. A bigger
lamp capacitor, for example, reduces the need for CM and LC tank to be adjusted to
supply a larger power gain. However, the larger power gain is at the cost of a higher
half-bridge current which leads to low-power efficiency and a narrow RMS operation
range.
(1) I
lamp
signal.
(2) RC signal.
(3) Half-bridge signal.
Fig 9.
CMP activated in boost state
