Abstract: L6598 is a control IC that integrates a resonant converter and a 600V high-voltage half-bridge driver on the same chip. It can replace single-ended resonant converters and multi-chip solutions, thereby effectively reducing the number of components and simplifying high-efficiency power supplies. design. The article introduces the internal structure, characteristics, functions, principles and application circuits of L6598.
Keywords: high voltage resonant controller, soft start timing, frequency setting, bootstrap drive, protection L6598
1 Overview
The high-voltage resonator L6598 officially launched by ST Company in February 2000 is the world's first control IC that integrates resonant conversion and a 600V high-voltage half-bridge driver on the same chip. It is manufactured using BCD (bipolar-CMOS-DMOS) offline (Off Line) technology, and the main line (Rail) voltage reaches 600V. It can be used as a high-efficiency power supply for AC/DC adapters, DC/DC modules, CTV and monitor systems with resonant topology. It can replace the previous half-bridge resonator composed of two chips, and can also replace the end-resonant converter. In addition, the price of L6598 is relatively low. When more than 10,000 pieces are used, the unit price of L6598 is only US$1. Therefore, using L6598 can not only simplify the power supply design, but also improve the performance-price ratio of the power supply.
2 Functional structure and working conditions
L6598/L6598D are available in 16-pin DIP and SO packages respectively. The pin arrangement is shown in Figure 1. Table 1 lists the L6598 pin functions.
Table 1 Pin names and functions of L6598
foot number | Name |
Function |
1 | css | Soft start capacitor terminal |
2 | Rfstart | Soft start frequency setting (low impedance voltage source) |
3 | Cf | Oscillator frequency setting |
4 | fR | Minimum oscillation frequency setting (low impedance voltage source) |
5 | OPOUT | Detect op amp output (low impedance) |
6 | OPIN- | Detecting op amp inverting input (high impedance) |
7 | OPIN+ | Detecting op amp non-inverting input (high impedance) |
8 | EN1 | Half-bridge blocking enabled |
9 | EN2 | Half-bridge non-blocking enable |
10 | GND | land |
11 | LVG | Low-side driver output |
12 | Vs | Supply voltage with internal Zener diode clamp |
13 | NC | not connected |
14 | OUT | High Side Driver Reference (Half-Bridge Output) |
15 | HVG | High-end driver output |
16 | VBOOT | Bootstrap supply voltage |
The typical start-up threshold of the L6598 power supply voltage Vs is 10.7V, the start-up current is 250μA, and the quiescent operating current is 2mA. The clamping voltage VCL on the Vs pin of L6598 is 15.6V (±1V), and the maximum supply current Is under VCL is 25mA. The maximum floating power supply voltage (VBOOT) on pin 16 is 618V, the output voltage (high-end reference) on pin 14 ranges from -1V to VBOOT-18V, the maximum low-end output voltage VLVG is 14.6V, and the maximum high-end output voltage VHVG The range is -1V~VBOOT. The maximum junction temperature of L6598 is 150℃, and the working environment temperature is -40℃+150℃.
The internal structure of L6598 is shown in Figure 2. It can be seen that L6598 mainly consists of a voltage controlled oscillator (VCO), a detection operational amplifier (OP AMP), two comparators with enable inputs, control logic, high /Composed of low-end driver, bootstrap driver, under-voltage blocking and soft-start circuit, etc.
Table 2 shows the recommended operating conditions for the L6598. In the table, the typical value of VCL is 15.6V. As long as VBOOT-VOUT is guaranteed to be <18V, the range of VOUT can be as wide as -3V~580V.
Table 2 Recommended operating conditions for L6598
parameter name | symbol | value | unit |
voltage | Vs | 10~VCL | V |
High-side reference (voltage) | V OUT | -1~(VBOOAT-Vcl) | V |
Floating supply mains voltage | VBOOT | 500 | V |
Maximum switching frequency | fMAX | 400 | kH 2 |
3 Functional principles
3.1 Soft start and oscillator
L6598 provides soft start function. The time it takes for the switching frequency to deviate from fstart at startup to the minimum value fmin is the soft-start period Tss. Figure 3 is a block diagram of the soft start and frequency offset circuit.
During soft-start, the current Iss charges the capacitor Css, producing a ramp voltage that is applied to the transconductance (gm) amplifier. As a result, the voltage signal is converted into a gradually increasing current after subtracting Ifstart. It can be seen that the L6598 soft start time depends on Css.
The working principle of current IOSC feeding to the oscillator is shown in Figure 4. IOSC is reflected (mirrored) twice by ×4 and ×8, generating a triangular wave on the oscillator capacitor CF. The oscillator frequency can be calculated by:
f min =1.41/R fmin G F
The above formula is relatively accurate in the frequency range of 30kHz to 100kHz.
3.2 Bootstrap driver
In order to charge the bootstrap capacitor CBOOT, this type of IC that was popular in the past required an external fast-recovery high-voltage diode DBOOT between the Vs pin and the VBOOT pin. The L6598 uses the bootstrap driver structure as shown in Figure 5, thereby omitting the external bootstrap diode DBOOT.
To drive synchronous DMOS, an internal charge pump is used to obtain a voltage much higher than Vs. The diode in series with the DMOS is used to prevent unwanted conduction and prevent current from flowing from VBOOT to Vs due to failure of the pump capacitor to completely discharge when the power is turned off quickly. When the low-end driver turns on CBOOT and is charged again, there will be a voltage drop on the bootstrap driver. This voltage drop is about 3V (the diode voltage drop is calculated as 0.6V).
The high/low-side driver provides optimal gate drive for external power MOSFETs or IGBTs, with driver currents sourcing 250mA and sinking 450mA respectively. The internal logic of the L6598 ensures a minimum dead time to avoid cross-conduction of the power MOSFET.
3.3 Operational amplifier
The operational amplifier in the L6598 provides low output impedance, wide bandwidth, high input impedance, and wide common-mode range, which are beneficial for protection or closed-loop control. Its output can be connected to the Rfmin pin to adjust the oscillator frequency.
3.4 Comparator
Two CMOS comparators are available to perform protection functions. The L6598 is able to recognize short pulses of 200ns width at the comparator input. If a threshold voltage pulse of 0.6V is detected at the ENI input terminal, the L6598 enters the latch-off state. At this time, the oscillator stops oscillating, and both LCG and HVG are low level. Once the fault is resolved, the device will resume normal operation. The EN2 input has a 1.2V threshold. Once the voltage on pin 9 reaches 1.2V, the comparator is triggered and the soft-start procedure is restarted.
4 Typical applications
Figure 6 shows the typical application circuit of L6598. The AC input voltage range of this circuit is 85~270V, which is suitable for international AC power supply standards. An active PFC boost pre-converter centered on L6598 is inserted between the bridge rectifier and the high-voltage aluminum electrolytic filter capacitor (C2). Its purpose is to generate a sinusoidal current at the AC input end so that the line power factor reaches 0.99 , and generate a constant 400VDC voltage at the output end of the PFC converter (both ends of C2).
The high-voltage resonant controller L6598 is used to drive the two switching tubes Q2 and Q3 in the circuit. Q2 and Q3 are turned on and off in turn, generating a square wave with a peak value of 200V that is rectified and filtered by transformer T1 to produce a DC output voltage (Vo). The resistor divider, TL431 and optocoupler form the feedback control loop from the secondary side to the primary side of the AC/DC adapter circuit transformer. One end of the primary end of the transformer is connected to the half-bridge output, and the other end (sometimes a capacitor is connected in series) is connected to the series capacitors C3 and C4. Using the coupling capacitor and the output filter inductor (L2) to form a (series) resonant circuit can make the charging of the coupling capacitor change linearly. Note that the resonant frequency must be lower than the switching frequency (fsw) of the power converter. Its resonant frequency is determined by the filter inductance and coupling capacitance reflected to the primary of T1.
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