Browse Prior Art Database

Multi-function Cell for AC/DC ICs

IP.com Disclosure Number: IPCOM000130310D
Published in the IP.com Journal: Volume 5 Issue 11A (2005-11-25)
Included in the Prior Art Database: 2005-Nov-25
Document File: 4 page(s) / 107K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

For some of off-line controller mains voltage sensing is necessary. For all of the off-line controllers startup unit is needed which charges the VCC capacitor before the IC starts to operate. Nowadays, start-up cell or start-up resistor is used which are shown in Fig. 1. The start-up cell is normally a depletion MOSFET which has a high channel resistor. Together with the high bus voltage this MOSFET operates as a current source. During start-up process the start-up cell is switched on and the capacitor at the pin VCC is charged by the MOSFET current. When the voltage at VCC pin reaches the turn-on threshold the IC is enabled and begins to operate. Thereafter the depletion MOSFET is switched off and the IC will be powered by an auxiliary winding which is coupled with the main inductor in case of PFC or the main transformer in case of an off-line PWM converter. This solution has disadvantages due to the high costs for the high voltage start-up cell, for assembly and the device package. As an alternative a start-up resistor is used. This solution is advantageous over the start-up cell due to the simple circuit and low costs for the controller IC, but is disadvantageous due to the losses during the whole operation. For off-line controller ICs the bus-voltage is sensed by a voltage divider for voltage regulation and protection functions (Figure 2). This voltage sensing circuit consumes certain power which is independent to the operation mode of the IC. In today’s solutions for PFC and PWM controllers the start-up resistor and the voltage sensing resistor are separated. In this approach the requirement on the low total power consumption during the standby mode operation of the IC can hardly be reached.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 42% of the total text.

Page 1 of 4

S

Multi-function Cell for AC/DC ICs

Idea: Dr. Jing Hu, SG-Singapore, Michael Herfurth, DE-Munic, Dr. Leo Lorenz, SG-Singapore

For some of off-line controller mains voltage sensing is necessary. For all of the off-line controllers startup unit is needed which charges the VCC capacitor before the IC starts to operate.

Nowadays, start-up cell or start-up resistor is used which are shown in Fig. 1. The start-up cell is normally a depletion MOSFET which has a high channel resistor. Together with the high bus voltage this MOSFET operates as a current source. During start-up process the start-up cell is switched on and the capacitor at the pin VCC is charged by the MOSFET current. When the voltage at VCC pin reaches the turn-on threshold the IC is enabled and begins to operate. Thereafter the depletion MOSFET is switched off and the IC will be powered by an auxiliary winding which is coupled with the main inductor in case of PFC or the main transformer in case of an off-line PWM converter. This solution has disadvantages due to the high costs for the high voltage start-up cell, for assembly and the device package. As an alternative a start-up resistor is used. This solution is advantageous over the start-up cell due to the simple circuit and low costs for the controller IC, but is disadvantageous due to the losses during the whole operation.

For off-line controller ICs the bus-voltage is sensed by a voltage divider for voltage regulation and protection functions (Figure 2). This voltage sensing circuit consumes certain power which is independent to the operation mode of the IC.

In today's solutions for PFC and PWM controllers the start-up resistor and the voltage sensing resistor are separated. In this approach the requirement on the low total power consumption during the standby mode operation of the IC can hardly be reached.

As mentioned above, the IC will be powered by an auxiliary winding during the normal operation. Figure 3 shows the normally used circuits by which the capacitor at pin VCC will be charged during off- (Figure 3a) and on-state (Figure 3b) of the switch, principally. Using these circuits it is hard to maintain the voltage VCC in all of operating modes. Using the circuit 3a the charging current is determined by the voltage of the auxiliary winding which depends on the secondary output voltage. This accordingly, there is no energy from the auxiliary winding for supplying the IC during the standby mode operation if the output voltage is lowered. Using the circuit 3b the charging current and therefore the voltage at the capacitor at VCC pin is dependent on the input voltage. To make the VCC voltage independent of the input voltage a certain circuit is needed. At high mains voltage the power consumption in this circuit is high which makes it difficult to achieve low total power consumption during the standby mode operation.

It is proposed to use the high-side resistor for VCC pre-charging and bus voltage sensing. This meth...