Overview
THX203H is a high performance current mode PWM controller. Designed for high-cost AC / DC converter. Up to 12W continuous output power is provided in the wide voltage range of 85V-265V, and the peak output power can be more than 18W. Optimized high-syndrome circuit design combines high performance ratio bipolar production processes to minimize the overall cost of products. The power controller can operate in a typical flyback circuit topology that constitutes a concise AD / DC converter. The startup circuit inside the IC is designed to be a unique current inhalation method, which can be started using the amplification of the power switch tube itself, which significantly reduces the power consumption of the start resistance; and the IC will automatically reduce the operating frequency when the output power is small. Thereby achieving extremely low standby power consumption. When the power tube is closed, the internal circuit reverse the power tube reversely biased directly with the CB high pressure resistance characteristics of the bipolar transistor, which greatly improves the high pressure of the power tube until 700V, which guarantees the safety of the power tube. . The IC also provides comprehensive anti-over-vehicle prevention and prevention, but also prevents the overload, the transformer saturation, the output short circuit, etc., improves the reliability of the power supply. The current limit and clock frequencies can be set by external devices. It is available in DIP8 standard packaging and environmentally-friendly-free packages that meet European standards.
Application information:
1, CT timing capacitance and switching frequency
The CT capacitor is performed by the internal current source to the rising edge of the constant current charging formation clock. When the charging voltage is 2.5V, the internal circuit will discharge the CT at a drop of 1.9 mA to the CT, form a lower edge of the clock, complete a clock cycle, A clock cycle is about:
T = CT * 24000 (s)
FS = 1 / t (Hz)
Although the bipolar circuit can operate at a higher frequency, it is still necessary to consider the impact of storage time on switching loss for bipolar power switches. It is usually a suitable switching frequency below 70 kHz. In a general application, the CT capacitance of THX203H can be configured at 680p, at which point the corresponding operating frequency is about 61 kHz.
2, FB feedback and control
In the normal working state, the voltage of the FB will determine the value of the maximum switching current, the higher the voltage, the larger the switch current (limited to peak current limits). The 600UA current source is pulled in the FB pin, and the pull-down electric resistance is about 33 kΩ (approximate equivalent value). In addition, when the FB voltage is less than 1.8V, the oscillating cycle will increase, the switching frequency is lowered, the more the switching frequency will be lower than the switching frequency. External FB capacitors will affect feedback bandwidth, which in turn affect certain external parameters, such as transient characteristics.
For the value of the CFB capacitor, the typical application can be selected between 10-100 NF, depending on the frequency characteristics of the feedback loop.
3, over temperature protection
The IC integrates an accurate over temperature protection. When the internal temperature of the chip reaches 140 ° C, the thermal protection circuit operates, pulling the clock signal to lower the switching frequency, reducing power consumption. The switching frequency is lowered with temperature, until the oscillator is turned off. As shown below:
4, power tube drive characteristics and high pressure-resistant bias technology
The power tube is driven by a ramp current, and the drive current increases with the output power, when FB = 0, the OB current is about 40mA, when FB = 6V, the OB current is about 120mA, and the driving power consumption at the time of small output is significant. reduce.
The IC is integrated into a unique bias technology. When the power tube is turned off, the OB output pulls down to the ground, and the offset OE outputs to about 1.5V, reverse biased emitting knots, accelerates the falling speed of the IC current, and expands A effective safety work area, the switch tube is subjected to the reverse CB voltage, so that the switching tube reaches 700 V voltage band. For more detailed switching pipe pressure resistance, please refer to the relevant technical data.
The bias waveform is shown below:
5, overvoltage and undervoltage protection
The IC has an undervoltage protection function with hysteresis. When the VCC voltage reaches 8.8V, the IC starts. This initial start voltage is provided, and the input high voltage is injected into the base of the switching tube by driving the resistance, and the enlarged IC current is charged with the VCC capacitor in the IC. Thereby, a drive voltage is formed. When the IC is working properly, the VCC voltage should be kept between 4.8-9V (including the full load output). If the VCC voltage drops to 4.4V, the oscillator will enter the closed state, and the VCC is further reduced to 3.8V, the IC begins. Restart. As shown below:
The IC internal VCC has a upper limit voltage comparator control. If the VCC is trying to be greater than 9.6V, the comparator action, the FB will be pulled, lock VCC to 9.6V, and reach the limit function of overvoltage. With this feature, it is convenient to facilitate the voltage feedback function of the front end, and the output voltage when the output is output is greatly increased, and the safety of the load can be avoided. Because this feature exists, the VCC design should be kept in a suitable range, avoiding the rise in VCC in the large output load, and the output voltage caused by the IC overvoltage limit.
6, maximum switch current limit
The IC has a cycle current limit function. Each switching cycle detects the switching current, and the current set of the current or the anti-upper current is entered, and the current detection has real-time elimination function, shielding switch spikes, and avoids error detection of switching current. Reasonable temperature compensation eliminates the effects of temperature, relatively conventional MOSFET (RON changes in temperature change) switch chip, switching current can be very accurate in a broad range, which will allow designers to design It is not necessary to leave too much margin to meet a large working temperature range, and improve the safety of the circuit.
For THX203H, the maximum switch current limit value allowed is 0.80A. In one design of the 80V reflective voltage, 0.65A switch current, the output power of greater than 12W can be easily achieved, and the width temperature range is satisfied.
7, cooling requirements
For a typical power switch, the necessary cooling measures should be used to avoid excessive temperatures leading to thermal protection. The main heat generation of the IC is the heat generated by the switching loss of the switch. Therefore, the appropriate heat dissipation position is the pin7-8 feet of the IC, and an easy-to-use method is to lay a certain area of PCB copper foil in PIN7-8 foot, especially in copper. Tinplate on the foil will greatly increase heat dissipation. For an 85-265V input, a typical application of 12W output, 200 mm2 copper foil area is necessary.
The reference wiring is shown below:
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