Innovative and high performance 8 pin LLC resonant controller

LCD TVs are now increasingly being accepted and preferred by customers, thanks to its several advantages such as high definition, bigger screen size and slimmer panel. Along with this, however, the LCD TV’s power supply designers are facing many design challenges, such as high energy efficiency and high power density requirements, lower EMI radiation, and low cost of design. In response to this, Infineon Technologies has released the ICE1HS01G, an 8-pin half bridge LLC controller that can help power supply designers to easily fulfill these various and strict requirements.

Infineon’s ICE1HS01G is specially designed for switch mode power supplies used in LCD/PDP TVs, AC/DC adapters and audio systems. The ICE1HS01G enables easy PCB layout and includes all necessary control strategies for the HB LLC resonant converter. The device allows the designer to choose the suitable operating frequency range by programming the oscillator with an external resistor. It has a built-in soft-start function, which limits both the inrush current and the overshoot of output voltage. In addition, the ICE1HS01G performs all necessary protection functions in HB LLC resonant converters.

Key specifications of the ICE1HS01G include: maximum 600kHz switching frequency; adjustable minimum switching frequency; 50 percent duty cycle; mains input under voltage protection with adjustable hysteresis; two levels of overcurrent protection: frequency shift and latch off; open-loop/over load protection with adjustable blanking time; built-in digital and nonlinear softstart; and adjustable restart time during over load protection.

Pin Configuration
- FMIN (Minimum Switching Frequency)
An external resistor RFMIN is connected between this pin and the ground. The voltage of this pin is constant during operation and thus the resistance determines the current flowing out of this pin. The minimum switching frequency is determined by this current. The maximum switching frequency during normal operation and the maximum switching frequency during soft start are both related to the current flowing out of FMIN pin.

- CS (Current Sense)
The current sense signal is fed to this pin. Inside the IC, two comparators are provided. If the voltage on CS pin is higher than the first threshold (0.8V typically), IC will increase the switching frequency to limit the maximum output power of the converter. If the voltage on this pin exceeds the second threshold(1.6V typically), IC will be latched off immediately.

- FB (Feedback)
This pin is connected to the collector of the external optocoupler. Internally, during normal operation, this pin is connected to reference voltage source with a pull-up resistor (RFB). The IC uses the voltage on this pin to adjust the switching frequency within the range of maximum and minimum frequency set by FMIN pin. If FB voltage is higher than VFBH for a certain internallyn fixed blanking time (20ms), an extended timer will be started. If over load/open loop protection exists longer than the extended blanking time, IC will enter auto-restart mode. Another off timer starts from the instant IC stops switching till IC starts another soft start. This off timer is determined by the resistors and capacitor connected to VINS pin.

- VINS (Mains Input Voltage Sense)
The mains input voltage is fed to this pin via a resistive voltage divider. If the voltage on VINS pin is higher than the threshold VINSon (1.25V typically), IC will start to operate with softstart when VCC increases beyond turn on threshold (12V typically). During operation, if the voltage on this pin falls below the threshold VINSon, IC will stop switching until the voltage at this pin increases again. When IC goes into over load protection mode, IC will stop switching and try to restart after a period of time. This period is adjustable by the RC network connected between VINS pin and ground.

GND (ground), IC common ground; LG (low side gate drive), low side power MOSFET driver; HG (high side gate drive), high side power MOSFET driver; and VCC, supply voltage to the IC.

Application Information
Minimum switching frequency
The minimum switching frequency is a very important factor to guarantee the LLC topology’s inductive mode operation and output voltage regulation at low line input and full load condition. The ICE1HS01G allows the minimum switching frequency easily programmed by connecting an external resistor RFMIN between FMIN pin and ground. The IC internal circuit provides a regulated 1.5V at FMIN pin. The resistor RFMIN, connected from FMIN pin to GND, determines the current (IFMIN) flowing out from FMIN pin. A certain current proportional to IFMIN is defined as the minimum charging current (Ichg_min), which in turn defines the minimum switching frequency. The relationship between minimum switching frequency and RFMIN is shown in Figure 2.

IC power supply and soft start
The ICE1HS01G is targeted at applications with auxiliary power supply. In most cases, a front-end PFC pre-regulator with a PFC controller is used in the same system. The ICE1HS01G starts to operate when the supply voltage VCC reaches the on-threshold, typically 12V. The minimum operating voltage after turn-on, VCCoff, is typically 11V. The maximum supply voltage VCCmax is 18V.

After the IC supply voltage is higher than 12V, and if the voltage on VINS pin is higher than 1.25V, the IC will start switching with soft start. The soft start function is built inside the IC with a digital manner. During softstart, the switching frequency of the MOSFET is controlled internally. The maximum duration of softstart is 32ms with 1ms for each step. Figure 3 illustrates the actual switching frequency VS start time when RFMIN=22kΩ. During softstart, the frequency starts from 250kHz, and step by step drops to normal operation point.

Output Voltage Regulation
The output load information is fed into the controller through feedback voltage VFB. Inside the IC, the feedback (FB) pin is connected to the 5V voltage source through a pull-up resistor RFB. Outside the IC, this pin is connected to the collector of optocoupler. If the output load is increased, and consequently VFB is higher, ICE1HS01G will reduce the switching frequency to regulate the output voltage and vice versa. The effective range of feedback voltage VFB is from 1V to 4V.

Over Current Protection
Current sense pin in ICE1HS01G is only for protection purpose. ICE1HS01G features two-level over current protection. In case of over-load condition, the lower OCP level (0.8V) will be triggered, the switching frequency will be increased according to the duration and power of the over load. The higher OCP level (1.6V) is used to protect the converter if transformer winding is shorted. When Vcs reaches 1.6V, the IC will be latched immediately.

Over Load/open loop Protection
In case of output over load or open control loop fault, the FB voltage will increase to its maximum level. If FB voltage is higher than VFBH and this condition last longer than a fixed blanking time of TOLP (20ms), the IC will enter auto-restart mode.

Mains under Voltage Protection
The working range of mains input voltage needs to be specified for LLC resonant converter. It is important for the controller to have input voltage sensing function and protection feature, which allow the IC to stop switching when the input voltage drops below the specified range and restart with soft start when the input voltage resumes to its normal level. Thanks to the internal current source Ihys (12µA) connected between VINS pin and Ground, an adjustable hysteresis between the on and off threshold of mains input voltage can be created as:

Open Load Protection
At very light load condition, the designed maximum frequency may not be high enough to regulate the output voltage, the output voltage may lose control and cause damages. To avoid this issue, the feedback signal VFB is continuously monitored. When VFB drops below VFB_off (typical 0.2V), the switching signal will be disabled after a fixed blanking time TFB (typical 200ns). VFB will then rise as Vout starts to decrease due to no switching signal. Once VFB exceeds the threshold VFB_on (typical 0.3V), IC resumes to normal operation.

Efficiency
Thanks to ICE1HS01G, an efficiency of 94.8 percent in the LLC stage can be achieved (Figure 4), which is the test result based on a 200W, 24V/12V output demoboard for LCD TV applications.

Infineon Technologies Asia Pte Ltd
www.infineon.com
References
1. ICE1HS01G datasheet, Infineon Technologies AG, 2008
2. RW ERICKSON, D MAKSIMOVIC: ‘Fundamentals of power electronics’ (Kluwer Academic Publishers, 2001), pp. 705–755

Caption

Figure 1: Pin configuration (top view).




Figure 2: Minimum switching frequency VS RFMIN.

Figure 3: Switching frequency during softstart at RFMIN=22kΩ.


Figure 4: LLC stage efficiency.