Overview of Operation

A project log for 350 Watt True Sine Inverter

Functional design revision of the prototype TS350.

brian-cornellBrian Cornell 05/08/2019 at 20:260 Comments

The TS350r1 is a two-stage inverter.  The first stage boosts the DC input voltage to greater than 200V DC depending on the input.  The second stage uses Direct Digital Synthesis (DDS) to generate a 60Hz sine wave between 100 and 120V AC RMS.  The AC output is fully isolated from the inverter and input.

Power Input

The unit is connected to a power source via Anderson PowerPole 45A PCB mounted connectors.  The (-) connects directly to inverter ground.  The (+) is connected to two parallel wired 30A MINI fuses (F201, F202) providing 60A protection for the unit.  The output of this parallel pair is connected to the input of a Solid State Relay (SSR) and a low power SPST on/off switch (S201).  The output tap of the switch is connected to a 1.6A fuse (F203) powering the start and inverter auxiliary power supply circuit.

Start / Auxiliary Supply

The start circuit provides over-volt protection, inrush limiting, and a stable supply voltage for the inverter logic & gate drive circuitry.  Two layers of over-volt protection are implemented.  The first is auto-resetting.  Q205 is a pass transistor that must be on for the auxiliary supply (and hence the unit) to function.  In normal operation it is biased on via Q206 & R205.  D203, R204, and Q203 form a voltage sensitive switch that, depending on tolerances, turns on (saturates) around 16.2V.  When this occurs the gate of Q206 is pulled low and it turns off.  In turn, R206 removes the negative bias from the gate of pass transistor Q205 and the auxiliary supply is turned off.

The second layer is a crowbar circuit formed by D202 and F203.  D202's value is set so that it enters full conduction by ~ 30V.  The excessive current flow opens F203 and unit repair is limited to fuse (and possibly D202's) replacement.

Inrush limiting functions thru the action of D204, R210 and the SSR.  During a normal power-on sequence the SSR remains off and energy reaches the inverter's significant bulk capacitance via D204 and R210.  A short in the inverter may open fuse F203; but if not R210 is a fusible resistor that is non-flammable.  During normal operation the SSR is turned on by the controller prior to starting the inverter.  D204 is necessary to prevent the auxiliary supply receiving power via the SSR once it is on and the power is switched off.

U201 is the auxiliary supply and consists of a Commercial Off The Shelf (COTS) DC-DC isolated SMPS capable of delivering a regulated 12V 500mA output.  This powers the gate drives, fans, and 5V logic LDO, U202.

Solid State Relay

A discrete, single pole, SSR is implemented for inrush management and to avoid the need for an expensive, bulky, switch capable of handling the full input current.  Q202 & Q204 implement the relay for the positive supply.  Gate bias is provided by a simple isolated flyback circuit.  It delivers a floating ~ 12V DC drive to the gates using a fixed duty cycle PWM provided by the controller.  Q201 switches the primary and C201 provides bulk capacitance.  Output recification & filtering is provided by D201 & C202.


The inverter operates at 50kHz as a DC transformer using a fixed on time (95% duty cycle) and a full bridge drive circuit for the transformer.  Monolithic half-bridge gate drivers utilizing bootstrap circuits for the high side floating supply are used.  High-side current sensing is provided by R302 and R303.  The differential voltage across them is amplified by U2 and level-shifted by Q1 for processing by the controller.  The controller can terminate the gate drive per-cycle and also derives RMS current & power.

The transformer consists of a single, 2-turn primary, 32-turn High Voltage (HV) secondary, and 3-turn auxiliary secondary.  To reduce Common Mode (CM) noise two faraday screens between the primary & HV secondary are included:  one connected to the inverter (-) and the other to the HV (-).

D401-404 and D405 form full-bridge rectifiers for the sine auxiliary & HV supplies respectively.  C405 & C406 provide bulk capacitance & decoupling for the auxiliary.  U403, R402, R403, C409, and C411 provide a regulated 13V DC for the sine gate drives and power for the 5V LDO, U404 that supplies the sine controller.

C407, C408, C410 provide bulk capacitance for the HV supply for the sine bridge.


Electrically, the sine bridge is configured like a conventional full bridge but functionally it operates like complementary synchronous buck power sections.  Each side, A or B, is on for a half-sine period with the high-side MOSFET driving its respective output L-C filter and that low-side MOSFET free-wheeling.  During this same half-cycle the opposite low-side MOSFET is on the entire time to provide a return path.  More detail on this in a separate log.

Except for component values the gate drive design is identical to the inverter (the same gate drivers are used).  

No over-current protection is necessary in the sine section.  For per-cycle it relies on the impedance of L401 & L402 to limit di/dt.  For longer-term events it relies on the limited bulk capacitance in the sine HV supply and the inverter's relatively high impedance & over-current protection features.

The AC output is made available to the user thru J401.  MOVs 401, 402, & 403 provide transient protection for the sine bridge from user appliances.

The sine bridge is driven by dedicated microcontroller U2.  It is responsible for responding to on/off commands from the main controller, U101, generating the DDS pattern, and temperature monitoring of the sine bridge.  The controller performs a crude form of output voltage regulation by sensing the auxiliary supply voltage and adjusting the DDS pattern.  This is necessary since the inverter's HV supply tracks the input voltage.

Thermal Management

At full power the unit generates approximately 68 watts of heat.  All switching MOSFETs and power rectifiers are secured to heatsinks and include faraday screens between their case and sink to reduce CM noise.  The main PCB includes two fans positioned between the sine & inverter sections that function to circulate air in a pull/push manner (sine->inverter).  A third fan is mounted to the inside of the front face plate to ensure fresh air circulates thru the case.

The fans are turned on & off by the main (inverter) controller based on the temperature of the inverter section (this is the hottest).  Temperature monitoring is done in the inverter & sine sections by the respective controllers.  Sensing is performed with Negative Temperature Coefficient (NTC) resistors RT301 & RT401.  Each are electrically & thermally connected to the mounting post of the heatsink expected to be the hottest in each section.

The sine bridge will shut down if its controller senses a hot condition and will signal the main controller by setting its ACK line FALSE.  The main controller will disable the unit and set a fault code if it detects a hot temperature directly or via the sine's ACK input.

Main Controller

U101 is the main controller and is responsible for the overall operation of the unit.  At power-on it performs diagnostic checks for temperature, voltage, and shorts (in the inverter) and will disable the unit with a fault code for any out-of-bound condition found.  Using on-board peripherals it drives the full-bridge and performs per-cycle current limiting without processor intervention.  It controls the on/off state of the sine section thru a simple CMD/ACK interface with galvanic isolation provided by U401.  It directly controls the SSR via PWM output to the flyback drive.

During normal operation its main function is to monitor the environment and maintain the load meter.  Abnormal conditions will result in the controller disabling the unit and displaying a fault code on the meter.

Transient & load management is an important function of the controller.  Transients & surges are normal for many AC appliances and cannot be avoided.  If the unit were to fault at the first indication of excessive current many devices would not work.  To overcome this the controller allows high current events for controlled periods of time and only disables the unit if they exceed time limits.

A service port is available for firmware updates, diagnostics, and tuning of some operating parameters.  X101 is a micro-USB jack that can be connected to a Microchip PICkit ICSP programmer or a TTL level RS232 interface (115.2kbps, 8 data bits, 1 stop, no parity).