High performance resonant mode controller MC33066-EEWORLD

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1. Introduction

MC33066 is a high-performance resonant mode controller launched by ON Semiconductor, USA, suitable for offline and DC-DC converters. The following briefly introduces the features, pin functions, electrical parameters and working principle of MC33066.

2 Features and Pin Description

2.1 Features

(1) Built-in variable frequency oscillator with a control range of 1000:1;
(2) Oscillator dead time can be programmed and controlled, and the cut-off time can be fixed;
(3) Built-in precision re-triggerable single-shot pulse timer;
(4) Built-in precision bandgap reference power supply, which can be fine-tuned;
(5) 5.0MHz error amplifier with output clamping function;
(6) Dual-channel high-current totem pole drive output circuit;
(7) Optional undervoltage lockout threshold;
(8) The controller has an enable terminal;
(9) The soft start circuit can be programmed;
(10) In offline converter applications, the controller has a low startup current.

2.2 Pin Description

MC33066 adopts two package types: PDIP-16 and SO-16. The following takes PDIP-16 as an example for introduction. Its pin arrangement is shown in Figure 1. The pin functions of MC33066 are as follows:

·Osc Deadtime (pin 1): Oscillator dead time setting terminal. This terminal is connected to the oscillator dead time setting resistor RDT.
·Osc RC (pin 2): Oscillator timing element access terminal. This terminal is connected to the timing resistor ROSC and timing capacitor COSC.
·Osc Control Current (pin 3): Oscillator maximum frequency limit terminal. This terminal is connected to the output terminal of the error amplifier through the resistor RVFO.
·GND (pin 4): Signal ground.
·Vref (pin 5): Precision 5V reference voltage output terminal.
·Error Amp Out (pin 6): Error amplifier output terminal. This terminal provides bias to pin 3 through the resistor RVFO.
·Error Amp Inverting Input (pin 7): Error amplifier inverting input terminal.
·Error Amp Noninverting Input (pin 7): Error amplifier non-inverting input terminal.
·Enable/UVLO Adjust (pin 9): Enable terminal/undervoltage lockout threshold adjustment terminal.
·Fault Input (pin 10): Fault detection signal input terminal.
·CSoft-Start (pin 11): Soft start capacitor access terminal. This terminal is connected to an external soft-start capacitor.
· Drive Output B (Pin 12): Drive output terminal B.
· Drive Gnd (Pin 13): Power ground.
· Drive Output A (Pin 14): Drive output terminal A.
· VCC (Pin 15): Bias power input terminal.
· One-Shot RC (Pin 16): One-Shot pulse timer external timing component access terminal. This terminal is connected to an external timing resistor RT and timing capacitor CT.

3. Rated parameters

The rated parameters of MC33066 are shown in Table 1.

4. Working Principle

MC33066 is a high-performance resonant converter controller suitable for offline or DC-DC converters. MC33066 uses frequency modulation to control the resonant conversion by fixing the on-time or off-time of the switch tube. The controller integrates a variable frequency oscillator with adjustable dead time, a single-shot pulse timer, a precision reference power supply with temperature compensation function, a high-gain broadband error amplifier with output clamping function, a trigger circuit, a soft start circuit, a fault detection circuit, an undervoltage lockout comparator, and a dual-channel high-current totem pole drive circuit, etc. Its principle block diagram is shown in Figure 2.

4.1 Main control loop

Under the combined action of the variable frequency oscillator, the one-shot pulse timer and the error amplifier, the MC33066 can control the output pulse width and repetition rate. The one-shot pulse timer is triggered by the oscillator to generate the corresponding drive pulse signal, which is input into the totem pole drive circuit through the T flip-flop. The error amplifier monitors the output voltage of the converter and adjusts the operating frequency of the oscillator accordingly. In the main control loop, high-speed Schottky logic circuits are used, which can minimize the transmission delay time and improve the high-frequency characteristics of the system.

4.2 Oscillator

The MC33066 uses a variable frequency control oscillator. The oscillator triggers the one-shot pulse timer and initializes the drive pulse. In addition, the initial voltage of the external capacitor of the one-shot pulse timer and the minimum dead time between the output drive pulses are determined by the variable frequency oscillator. The operating frequency of the oscillator in the MC33066 can exceed 1MHz. The error amplifier controls the oscillator frequency range by 1000:1, and as long as the external components are properly selected, it is easy to determine the minimum and maximum operating frequencies of the variable frequency oscillator, and the oscillator frequency can be well programmed. The oscillator also has the feature of adjusting the dead time, which can flexibly control the size of the dead time between the output drive pulses.
Figure 3 shows the schematic diagram of the oscillator and the single-shot pulse timer. Transistor Q1 charges the oscillator's external capacitor COSC through an external resistor RDT. When the voltage on COSC exceeds the upper limit voltage of 4.9V of the oscillator comparator, the base of the transistor will be pulled down to a low level, and COSC will be discharged through the external resistor and the current source mirror built into the controller. When the voltage on COSC drops to the lower limit voltage of 3.6V, transistor Q1 is turned on again and COSC starts charging again.

[page] Note that the external resistor RDT is optional. If this resistor is not connected, that is, RDT is equal to 0, the time for capacitor COSC to charge from 3.6V to 5.1V will be less than 50ns. Due to the fast conversion rate and the influence of the propagation delay time of the comparator, it is not conducive to the control of the peak voltage of the oscillator. For this reason, transistor Q2 is added to the base of transistor Q1, and Q1 is connected to the 5.1V reference voltage through Q2. In this way, the peak voltage waveform of the oscillator is accurately limited to 5.1V.
The control of the oscillator oscillation frequency is achieved by changing the control current IOSC flowing through the resistor RVFO. The control current IOSC flows into the oscillator maximum frequency limit terminal (pin 3). Under the action of this control current, the unity gain current mirror draws the same amount of current from capacitor COSC. As IOSC increases, the discharge process of COSC also accelerates. In this way, the oscillation period decreases accordingly, and the oscillation frequency increases accordingly. When the output voltage of the error amplifier reaches the upper limit of the clamping voltage, that is, about 2.5V higher than the voltage on pin 3, the oscillation frequency reaches the maximum value. At this time, the minimum discharge time of capacitor COSC is as shown in formula (1).

When the output voltage of the error amplifier is lower than the bias voltage of the current mirror, the oscillation frequency reaches the minimum value when the control current IOSC drops to zero. At this time, the capacitor COSC will discharge through the external resistors ROSC and RVFO. The maximum discharge time of the capacitor COSC is shown in formula (2).

At any time, as long as transistor Q1 charges capacitor COSC, the output of controller MC33066 will be in the off state. The size of the dead time between the output drive pulses can be adjusted by controlling the charging time of capacitor COSC. Adding resistor RDT can reduce the charging current of capacitor COSC, so that the charging time of capacitor COSC is extended, increasing the dead time between the output drive pulses. If the resistance value of RDT varies between 0Ω and 1000Ω, when the size of capacitor COSC is 300pF, the dead time range will be between 80ns and 680ns. At this time, the oscillator charging time is shown in formula (3).

The appropriateness of the values of resistors ROSC and RVFO has a great influence on the programming control of the minimum and maximum frequencies of the oscillator. After the resistor RDT is determined according to the size of the dead time, the minimum operating frequency of the oscillator is determined by the resistor ROSC, as shown in the following formula:

Similarly, the maximum operating frequency of the oscillator is determined by the resistor RVFO, as shown in the following equation:

The value of resistor RDT will affect the peak voltage of the oscillator. When the resistance RDT gradually increases from zero, the time required for the capacitor to charge COSC will gradually increase. Therefore, the overshoot of the upper threshold will be alleviated, and the peak voltage of the oscillator will also drop from 5.1V to 4.9V. Of course, when the resistance RDT is zero, the accuracy of the oscillation frequency is the best.

4.3 Single-shot pulse timer

While Q1 is charging the oscillator external capacitor COSC, the external capacitor CT of the single-shot pulse timer will also be charged, see Figure 3. When Q1 is turned off by the oscillator comparator, the single-shot pulse cycle begins, and the capacitor CT will discharge through the resistor RT. When the voltage on CT drops to the threshold voltage of the single-shot pulse comparator, the single-shot pulse cycle ends. The voltage on capacitor CT is discharged from the initial value of 5.1V to 3.6V, and the single-shot pulse cycle tOS is shown as follows:

The factors that affect the single-shot pulse period are mainly the threshold voltage error and the transmission delay time. The output signals of the oscillator comparator and the single-shot pulse comparator generate a pulse signal tON after passing through the "NOR gate", and the pulse signal drives the T trigger and the drive circuit. When the discharge time of the oscillator exceeds the period of the single-shot pulse, the length of tON is equal to the length of the single-shot pulse period tOS. If the oscillator discharge time is less than the single-shot pulse period, the oscillator comparator will interrupt the pulse signal tON and re-trigger the single-shot pulse timer. See Figure 4 for the relevant timing waveforms. The timing waveform on the left side of the figure shows the situation when the on-time is fixed and the off-time changes, while the timing waveform on the right side shows the situation when the off-time is fixed and the on-time changes after the single-shot pulse timer is re-triggered.

Figure 4 Related timing waveform

4.4 Error Amplifier

The error amplifier in MC33066 is internally compensated, and its DC open-loop gain exceeds 70dB, and the input offset voltage is less than 10mV, ensuring that the minimum gain-bandwidth product can reach 2.5MHz. The common-mode voltage range of the error amplifier is extended to 1.5V~5.1V, covering the reference voltage. If the common-mode voltage is lower than 1.5V, the output signal of the error amplifier will be set to a low level to provide the lowest oscillation frequency.
The output voltage of the error amplifier provides a bias to the oscillator's maximum frequency limit terminal (pin 3) through the resistor RVFO. In order to suppress the fluctuation of the error amplifier output voltage, a clamp circuit is added to the output of the error amplifier to limit the maximum oscillation frequency of the oscillator. Under the action of the clamp circuit, the voltage on the resistor RVFO is limited to 2.5V, so that the current IOSC is limited to 2.5V/RVFO. See Figure 5 for the schematic diagram of the error amplifier and the clamp circuit.

4.5 Driver Output Circuit

The schematic diagram of the driver output circuit in MC33066 is shown in Figure 6. The totem pole driver output circuit shown in the figure can provide up to 1.5A of sink current or source current, and can directly drive power MOSFET. When driving a 1.0nF capacitive load, the typical value of the rise time and fall time of the drive pulse is 20ns.

[page]4.6 Undervoltage Lockout and Reference Power

Supply The undervoltage lockout comparator monitors the input bias voltage VCC and the reference power supply, as shown in Figure 7. When VCC exceeds the upper threshold of the undervoltage lockout comparator, the VCC undervoltage lockout comparator will put the reference power supply in an effective working state. When the voltage on pin 5 rises to 4.2V, the Vref undervoltage lockout comparator will reset the undervoltage lockout signal to a logic 0 state, and the main control circuit will start working. If VCC falls below the lower threshold of the undervoltage lockout comparator, the VCC undervoltage lockout comparator will stop the reference power supply from working. The Vref undervoltage lockout comparator will then set the undervoltage lockout signal to a logic 1 state, stopping the controller from working. The threshold voltage
of the VCC undervoltage lockout comparator can be set through the enable terminal/undervoltage lockout threshold adjustment terminal (pin 9). If this terminal is open, the VCC undervoltage lockout comparator will start the controller at 16V and shut down the controller at 9V. If this terminal is connected to VCC, the upper and lower thresholds of the VCC undervoltage lockout comparator will be reduced to 9V and 8.6V respectively. If this terminal is set to a low level, the input of the VCC undervoltage lockout comparator will be pulled down to a low level by the internal diode of the controller, and the controller will be shut down.
The reference power supply can provide a precise 5.1V reference voltage and can also provide a 10mA drive current to the external winding. In addition, the reference power supply has an active short-circuit protection function with an accuracy of 2%.

4.7 Fault Detection Circuit

The high-speed fault comparator and latch circuit inside MC33066 are shown in Figure 8. The fault detection signal input terminal is connected to the input terminal of the fault comparator. If the detection signal on this terminal exceeds the 1.0V threshold of the fault comparator, the fault latch circuit will be set and the main control circuit will be disabled. The output terminal of the fault comparator is directly connected to the drive output circuit. In this way, the transmission time of the fault signal can be shortened. At this time, the transmission time from the fault detection signal input terminal to output terminal A and output terminal B can reach 70ns. The output terminal of the fault latch circuit is "OR" with the output signal of the Vref undervoltage lockout comparator to generate the corresponding undervoltage lockout and fault mixed signal. This signal makes the oscillator and the single-shot pulse timer in a disabled state by continuously charging the capacitors COSC and CT.
During the startup process, the fault latch circuit is reset by the logic 1 signal at the output terminal of the Vref undervoltage lockout comparator. In addition, when the voltage on pin 9 is pulled down to a low level and the reference power supply is disabled, the fault latch circuit is also reset.

4.8 Soft-start circuit

The principle diagram of the soft-start circuit is shown in Figure 8. Under the action of the soft-start circuit, the variable frequency oscillator starts working at the lowest frequency, and then the frequency gradually increases until it meets the requirements of the feedback controller loop. In the initial stage, the external capacitor CSoft-Start on the soft-start capacitor access terminal is discharged under the action of the undervoltage lockout and fault mixed signal. When the voltage on the capacitor is low, the output of the error amplifier is kept at a low level through the soft-start buffer circuit. When the undervoltage lockout and fault mixed signal become logic 0, the 9.0μA current source inside the controller starts to charge the soft-start capacitor CSoft-Start. If the soft-start function is not required, pin 11 can be left open.

5 Conclusion

Figure 9 shows the complete principle block diagram of MC33066. MC33066 is suitable for the control of series resonant, parallel resonant or half-bridge/full-bridge resonant converters. Its operating modes include discontinuous conduction mode (DCM), continuous conduction mode (CCM) or discontinuous conduction and continuous conduction mixed mode, and is suitable for double-ended push-pull converters or single-ended converters. For example, in a parallel resonant converter, if it works in discontinuous conduction mode, MC33066 can use a variable frequency operation mode with a fixed on-time and a variable off-time; if it works in continuous conduction mode, MC33066 can use a variable frequency operation mode with a fixed off-time and a variable on-time.
For a wide input switching converter, when the input voltage is high, the parallel resonant converter can use a discontinuous conduction mode; when the input voltage is low, it can use a continuous conduction mode. At this time, the on-time is constantly changing in the discontinuous mode. The dead time is mainly used to provide the off-time required in the continuous conduction mode. If the frequencies of the discontinuous conduction mode and the continuous conduction mode cover the entire frequency range, then in the low frequency band, MC33066 will work in a variable frequency operation mode with a fixed on-time and a variable off-time. When the single-shot pulse timer is retriggered, MC33066 will switch to a variable frequency operation mode with a fixed off-time and a variable on-time. At this time, in the high frequency band, the converter will work in a continuous conduction mode.

Reference address：High performance resonant mode controller MC33066

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