L6599 Half bridge SMPS — Russian Translated Version — Resonant Converter Design Guidance. PART 1 Fundamental
L6599 Half Bridge Resonant controller.
L6599 is a half-bridge resonant converter controller with an integrated high-voltage driver, programmable oscillator, and fixed dead time.
It has a built-in circuit that replaces the high-voltage boost diode. The microchip is relatively new, based on the datasheet dated 2009.
Its latest version, L6599AT, used in the power supply unit described in this article, is dated 2017.
The L6599 microchip has several versions: L6599 — the very first version of the microchip, which is currently discontinued is L6599A — the first improved version that replaced L6599; L6599AF — a modification of the microchip designed for extreme temperatures; L6599AT — the most modern version of the microchip.
All of the listed versions of the microchip are compatible. In the described circuit, any of the listed versions of the microchip can be used, but it is recommended to use the latest and most modern version — L6599AT.
The microchip has a rich set of features that provide developers with extensive opportunities to build high-quality, modern switching power supplies based on it.
List of features of the L6599 microchip:
Micro-power startup;
Powerful built-in low and high-side driver;
Built-in boost diode for floating high-voltage driver supply;
Programmable oscillator;
Fixed dead time;
Soft-start;
Burst mode operation;
Interface for communication with power factor correction (PFC) controller;
Two-stage overload and short-circuit protection: frequency shift and forced shutdown;
Additional pin for stopping the microchip according to an external condition;
Protection against low input line voltage;
L6599 pins and their functions.
1 (Css): Pin designed for connecting an external soft-start capacitor, the capacitance of which determines the duration of operation in soft-start mode;
2 (DELAY): Pin designed for setting the delay time until shutdown in case of overload by current and time, after which the operation of the microcircuit will be resumed, after the current protection has tripped;
3 (CF): Pin designed for connecting an external timing capacitor that sets the minimum and maximum operating frequencies, as well as the frequency of soft-start;
4 (RFmin): Pin designed for setting the minimum and maximum operating frequencies, frequency of soft-start, and connecting the feedback circuit;
5 (STBY): Pin for connecting an external circuit that sets the conditions for switching to a packet mode;
6 (ISEN): Pin designed for connecting a current sensing circuit for monitoring the primary current of the converter, organizing current protection against overload and short circuit;
7 (LINE): Pin designed for controlling the voltage level on the high-voltage power bus of the converter, organizing protection against low input network voltage;
8 (DIS): Output pin for connecting an external circuit that stops the converter operation according to its conditions.
9 (PFC_STOP): Output pin for synchronizing the L6599 with the power factor correction (PFC) controller.
10 (GND): Signal and power ground pin for the lower-level driver.
11 (LVG): Output pin for the lower-level driver. Internally pulled down to GND.
12 (Vcc): Power supply pin for the L6599 (logic and lower-level driver).
13 (N.C.): Not connected pin. It is not electrically connected to any internal circuitry of the L6599.
14 (OUT): High voltage floating ground output pin for the upper-level driver.
15 (HVG): Output pin for the upper-level driver. Internally pulled down to OUT.
16 (VBOOT): Power supply pin for the high voltage upper-level driver.
Point to notice before start build SMPS with L6599
Some important parameters of L6599AT (may slightly differ for other modifications of the chip):
Maximum allowable supply voltage — 16 V;
Positive threshold voltage of the chip supply voltage (the voltage at which it transitions from the ULVO state to active mode) — 10 … 11.4 V;
Negative threshold voltage of the chip supply voltage (the voltage at which it transitions from active mode to UVLO state) — 7.5 … 8.9 V;
Start-up current — no more than 250 μA;
Operating current — no more than 5 mA;
Reference voltage of the first comparator of the ISEN output (overload protection) — 0.76 … 0.84 V;
Reference voltage of the second comparator of the ISEN output (short circuit protection) — 1.44…1.56 V;
Reference voltage of the LINE output comparator (low output voltage protection) — 1.2…1.28 V;
Voltage of the LINE output stabilizer (protection of the LINE output from high voltage) — 6…8 V;
Current hysteresis of the LINE output — 10…16 µA;
Reference voltage of the DIS output comparator (shut off by external condition) — 1.78…1.92 V;
Dead time — 0.2…0.4 µs;
Reference voltage of the RFmin output — 1.8…2.2 V;
Maximum current of the RFmin output — 2 mA;
Threshold voltage of the STBY output — 1.2…1.28 V;
Maximum allowable driver leakage/sink current +300/-700 mA.
Next, we will examine the operation of individual nodes and modes of the microchip.
UVLO (Under-Voltage Lockout): This is the state of the microchip when its power voltage is below a certain positive or negative threshold, as well as under other conditions that will be discussed later.
In UVLO mode, the microchip’s generator and driver are turned off, and the microchip itself consumes an extremely low current.
Generator: It is programmed using an external capacitor Cf connected to the corresponding output. The capacitor is alternately charged and discharged by a current whose magnitude is programmed using an external circuit connected to the RFmin output.
The voltage at RFmin is maintained with high accuracy, and the maximum current that can be provided by the specified output is 2 mA. The higher the instantaneous current flowing from the RFmin output and through the frequency-setting resistors to ground, the higher the generator’s operating frequency. The simplified internal circuit of the L6599AT microchip generator is shown in the figure.
The capacitance of capacitor Cf is chosen based on the current capabilities of the RFmin output. The values of the frequency-setting resistors Rfmin, Rfmax, and Rss are selected so that the generator can cover the entire frequency range necessary to maintain stable output voltage over the entire range of power supply voltages and loads on its output.
To calculate the resistance values of the frequency-setting resistors Rfmin and Rfmax, the following formulas are provided in the datasheet:
It is worth noting separately that when using the L6599 chip in the burst mode (more details on it later), the formula for calculating the resistance of the Rfmax resistor will be different and will have the following form:
However, these and subsequent formulas presented in this article are unlikely to be needed by you, as at the end of the article you will find a calculator created by the author for automatic calculation of all necessary values (more on this later).
The following illustration shows the time dependence between the voltage on the timing capacitor Cf, the voltage on the gate driver outputs, and the floating ground voltage of the upper level driver. Note that the voltage on the output of the lower level driver appears when the capacitor Cf is charged, and on the output of the upper level driver when Cf is discharged.
Thus, when starting the power supply or at the beginning of the next cycle of operation in burst mode, the lower level MOSFET always opens first. This is done to ensure that the voltage-boosting capacitor of the floating power supply of the upper level driver, which is charged every time the lower level MOSFET is opened, is fully charged before it is time to open the upper level MOSFET.
Batch / burst mode operation. When the resonant converter is lightly loaded or not loaded at all, it operates at maximum frequency. In these conditions, in order to maintain the output voltage at the specified level and to avoid hard switching, a significant residual magnetization current must flow through the primary winding of the transformer.
This current creates certain losses that prevent the converter from being economical. To solve this problem, the L6599 includes the ability to operate in batch mode.
In this mode, pulses to the key transistors are not continuous but are delivered in a limited number of series (packets) separated by long periods of inactivity when both switches are in the off state. As a result, the average residual magnetization current and the associated losses at idle will be significantly reduced.
Control of the batch /burst mode is achieved through pin 5 (STBY) of the chip. If the voltage on this pin drops below a threshold value, the chip enters a standby state: the chip’s oscillator is stopped, both key transistors are closed, and the soft-start capacitor remains charged.
The oscillator is reactivated as soon as the voltage on the STBY pin exceeds the threshold value by 50 mV, after which the next batch mode cycle begins, or the converter switches to normal mode (when the output load increases above the specified value).
To implement batch mode in the converter, the STBY pin must be included in the feedback loop. The following illustration shows the simplest way to implement batch mode, suitable for a narrow range of input voltages (with a PFC circuit):
In the case of a sufficiently large range of input voltages (without a PFC circuit in the converter scheme), it is necessary to use the circuit shown in the following illustration:
Here we should make a small digression. In this illustration (taken from the 2009 L6599 datasheet), there is a typo — the designations of the RC and RD elements are reversed. In fact, the resistors should be labeled the other way around. This typo was noticed and corrected in the 2017 datasheet:
This is not the only typo that the article author found in the L6599 datasheet from 2009. Other typos will also be indicated later. Be careful not to make a mistake.
So, the correct scheme for implementing the burst mode for a wide range of input voltages is shown in the following corrected illustration by the author
Since the switching frequency in a resonant converter depends on the input voltage, in the case of a wide range of input voltages, the power level at which the converter switches between the packet mode and normal mode would vary significantly.
Therefore, to avoid this issue, the circuit shown above adds information about the voltage on the high-voltage power bus of the converter to the voltage applied to the STBY pin.
Unfortunately, due to the highly nonlinear relationships between the switching frequency and the input voltage, the optimal ratio of resistors RA and RB will need to be determined experimentally. It is essential to strictly adhere to the relationship RA + RB >> RC.
Regardless of the specific circuit used to implement the burst mode operation, its principle of operation can be described as follows. When the load drops below a certain set value, the switching frequency will tend to exceed the maximum value set by the Rfmax resistor. This will cause the voltage on the STBY pin to drop below the threshold value and the microchip oscillator will stop, closing both key transistors.
After that, the voltage on the STBY pin will begin to increase due to the feedback circuit response to the cessation of energy supply to the secondary part of the power supply, and when the voltage on the STBY pin exceeds the threshold value by 50 mV, the microchip will restart and resume switching. After some time, the voltage on the STBY pin, in response to excess incoming energy, will again drop below the threshold value and the generator will stop again.
PFC_STOP. To simplify the designer’s work in achieving energy-saving requirements in power supplies with PFC, the L6599 offers the possibility of synchronizing its operation with the controller of the PFC system. This is done through pin 9 of the IC (PFC_STOP). This allows the PFC system to be turned off during operation in discontinuous mode, which reduces standby power consumption.
The following timing diagram illustrates the principle of operation of the discontinuous mode and the voltage at the PFC_STOP pin:
DIS. The microchip is equipped with an additional comparator, one of whose inputs is accessible from the outside — pin 8, and the other input of the comparator is internally tied to a reference voltage source of about 1.85 V.
When the voltage on the DIS pin exceeds the comparator reference voltage, the microchip immediately turns off, and its consumption is reduced to a very low value. In this state, the microchip remains until its supply voltage drops below the negative threshold value for resetting the trigger.
The presence of this pin allows the developer to implement many additional functions, such as protection against overheating, protection of the speaker system from DC voltage on the output of the Class D amplifier, and others.
LINE. The function of protection against low input voltage is implemented using the LINE pin. When the input mains voltage and the associated voltage on the high-voltage bus of the converter fall below the specified value, the microchip stops and automatically restarts when the input voltage returns to the specified range.
To control the converter’s input voltage, the LINE pin, via a voltage divider, can be connected to either the rectifier output or the KKM output. In the latter case, the function will additionally ensure that the converter is turned on and off in the correct sequence.
The LINE pin is internally connected to one of the inputs of a comparator, and the other input of the comparator is connected to a 1.25 V reference voltage source.
If the voltage on the LINE pin drops below the comparator’s reference voltage, the microchip’s oscillator will stop. This will result in a quick discharge of the soft-start capacitor and the KKM controller (if present in the power supply circuit and connected to the L6599 microchip) will also stop.
The low input voltage protection function is implemented using the LINE pin. When the input network voltage, and the associated voltage on the high-voltage bus of the converter drops below a certain value, the microchip stops and automatically restarts when the input voltage returns to the specified range.
To control the input voltage of the converter, the LINE pin, through a voltage divider, can be connected to either the rectifier output or the cash register output.
In the latter case, the function will additionally ensure the turning on and off of the converter in the correct sequence. The LINE pin is internally connected to one of the comparator inputs, while the other comparator input is connected to the 1.25 V reference voltage source.
If the voltage on the LINE pin drops below the comparator reference voltage value, then the microchip generator will be stopped. This will cause a quick discharge of the soft-start capacitor and also stop the cash register controller (if it is present in the power supply block diagram and connected to the L6599 microchip).
The generator operation will be resumed when the voltage on the LINE pin exceeds the comparator reference voltage value. The comparator is equipped with hysteresis with a current source of 13 µA, which is active as long as the voltage on the LINE pin is below the comparator reference voltage.
This provides an additional degree of freedom — it allows setting various turn-on and turn-off thresholds for the converter at low input voltage using a single external voltage divider. As an additional safety measure, for example, in case of damage to the voltage divider or abnormally high input voltage, if the voltage on the LINE pin exceeds 7 V, then the microchip will turn off.
It should be noted that the LINE pin is an input with a very high impedance, so it is very susceptible to interference, which can affect the protection threshold. To prevent this, a capacitor of 1–10 nF should be connected between the LINE pin and ground. If low input voltage protection is not required, then the LINE pin should provide a stable voltage ranging from 1.25 to 6 V.
The following timing diagram demonstrates the low input voltage protection principle in a clear and concise manner:
Here is another lyrical digression. The datasheet for 2009 provides the following formulas for calculating the triggering thresholds for this protection:
The formulas contain typos: the Rh underlined in red should be Rl. This typo was corrected in the datasheet for 2017. The correct formulas for calculation are as follows:
However, as with the previously provided formulas, you are unlikely to need them due to the availability of a calculator for automatic calculation created by the author.
Bootstrap. The provision of floating power to the high-side driver is provided using a bootstrap circuit. Typically, this requires the use of a fast high-voltage diode (Dboot) designed for charging the bootstrap capacitor (Cboot). In the L6599 IC, there is a patented internal circuit that replaces this external diode. The circuit is a synchronous rectifier that is controlled synchronously with the low-side driver.
This voltage boost circuit has a certain voltage drop during the charging of Cboot (when the lower level switch is open), which increases with the increase of the operating frequency.
At low frequency, the voltage drop is very small and therefore can be neglected, but at higher frequency it must be taken into account. The voltage drop of the upper level driver power supply leads to a decrease in the amplitude of the control pulse at the gate of the corresponding transistor, which can lead to a significant increase in the resistance of its open channel, and therefore to increased conduction losses.
This problem applies to powerful converters operating at a resonant frequency of more than 150 kHz. To avoid this problem and be able to operate at a high switching frequency at high output power, an external super-fast diode (Dboot) is required.
The value of the voltage drop across the L6599 voltage boost circuit can be calculated using the following formula:
The voltage drop on the L6599 voltage multiplier circuit during charging of Cboot (when the lower-level switch is open) is proportional to the working frequency and increases as the frequency increases. At low frequencies, the voltage drop is negligible and can be ignored, but at higher frequencies, it needs to be taken into account.
The voltage drop on the supply of the upper-level driver leads to a decrease in the amplitude of the control pulse at the gate of the corresponding transistor, which can significantly increase the resistance of its open channel and therefore increase conduction losses.
This problem is relevant for powerful converters operating at a resonant frequency above 150 kHz. To avoid this problem and be able to operate at a high switching frequency at high output power, an external super-fast diode (Dboot) must be used.
The magnitude of the voltage drop on the L6599 voltage multiplier circuit can be calculated using the following formula:
Where Qg is the charge of the key transistor gate, Rds is the resistance of the open channel of the voltage multiplier key transistor (150 ohms), Tcharge is the time in which the upper-level driver is in the ON state, which is approximately equal to half the switching frequency period minus the dead time, and Vf is the voltage drop on the diode connected in series with the voltage multiplier key transistor (approximately 0.6 V).
PART 2 WILL TALK EXPLAIN THE CONSTRUCTION OF LLC CONVERTER
