By Roy Reed,2014-12-23 17:31
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     High Precision CC/CV Primary-Side PWM Power Switch


     GS288D is a high performance offline PWM Power switch for low power AC/DC charger and adaptor applications. It operates in primary-side sensing and regulation. Consequently, opto-coupler and TL431 could be eliminated. Proprietary Constant Voltage (CV) and Constant Current (CC) control is integrated as shown in the Fig.1. In CC control, the current and output power setting can be adjusted externally by the sense resistor RS at S pin. In CV control, multi-mode operations are utilized to achieve high performance and high efficiency. In addition, good load regulation is achieved by the built-in cable drop compensation. Device operates in PFM in CC mode as well at large load condition and it operates in PWM with frequency reduction at light/medium load. GS288D offers power on soft start control and protection coverage with auto-recovery features including Cycle-by-Cycle current limiting, VCC OVP, VCC clamp and UVLO. Excellent EMI performance is achieved with frequency jitter technique.


     5% Constant Voltage Regulation, 5% Constant Current Regulation at Universal AC input Primary-side Sensing and Regulation Without TL431 and Opto-coupler Power on Soft-start Built-in Leading Edge Blanking (LEB) Cycle-by-Cycle Current Limiting VCC Under Voltage Lockout with Hysteresis (UVLO) Programmable CV and CC Regulation Adjustable Constant Current and Output Power Setting Built-in Secondary Constant Current Control with Primary Side Feedback Built-in adaptive current peak regulation Built-in Primary winding inductance compensation

     Fig.1. Typical CC/CV Curve Program cable drop compensation VCC OVP and VCC Clamp Pb-free DIP-8L


     Cell Phone /Digital Cameras Charger Small Power Adaptor Auxiliary Power for PC, TV etc. Linear Regulator/RCC Replacement







     High Precision CC/CV Primary-Side PWM Power Switch


     Pin Assignment(DIP8)

     Pin Description Pin Num Pin Name 1 VCC 2 FLTR

     3 4 5/6 7/8 COMP S DR G

     Description Power Supply The error amplifier output filter The

    voltage feedback from auxiliary winding. Connected to resistor divider

    from auxiliary winding reflecting output voltage. PWM duty cycle is

    determined by EA output and current sense signal at pin 4. HV MOSFET

    Source Pin. Current sense input. HV MOSFET Drain Pin. The Drain pin

    is connected to the primary lead of the transformer Ground

     Typical Application







     Block Diagram

     High Precision CC/CV Primary-Side PWM Power Switch

     VG 1 DR 5/6

     Current Reference

     Soft Driver

     Power MOSFET

     R S

     Q Soft-start

     burst mode Internal supply OSC



     Slope compensation


     S 4

     VCC 2




     VCC clamper

     Burst mode

     PWM Compartor



     COMP 3

     7/8 G

     Simplified Internal Circuit Architecture

     Absolute Maximum Ratings

     Parameter Value

     DR Voltage (off state) VCC Voltage VCC Zener Clamp Continuous Current FLTR Voltage S Input Voltage COMP Input Voltage Min/Max Operating Junction Temperature TJ Min/Max Storage Temperature Tstg Lead Temperature (Soldering, 10secs)

     -0.3V to BVdss -0.3 to VCC_clamp 10 mA -0.3 to 7V -0.3 to 7V -0.3 to 7V -20 ?æto 150?æ -55 ?æto 150?æ 260?æ

     Note: Stress beyond those listed under ??absolute maximum ratings?? may cause permanent damage to the device.

     Exposure to absolute maximum-rated conditions for extended periods may affect device reliability.

     Output Power Table





     230VAC ?À15% Adapter1 13W

     85-265VAC Adapter 1 12W

     Table 1. Notes: 1. Minimum continuous power in a typical non-ventilated enclosed adapter measured at 50 ?ãC ambient. Use of an external heatsink will increase power capability. 2. Minimum peak power capability in any design or minimum continuous power in an open frame design (see Key Application Considerations). 3. Packages: P: DIP8.







     High Precision CC/CV Primary-Side PWM Power Switch

     Min 13.5 Typ 14.5 Max 16.0 Unit V

     Electrical Characteristics

     (Ta=25?ã unless otherwise noted, VCC = 16V) C Symbol Parameter Test Conditions Supply Voltage (VCC) Section Turn-On Threshold VCC-ON VCC rising Voltage Turn-Off Threshold Voltage, Under Voltage VCC-OFF VCC falling Lockout on VCC(UVLO) ICC_ST VCC Start up Current VCC=13V Operation supply current ICC_OP Operation Current COMP=2V,S=0V, VCC =20V Over voltage protection Ramp up VCC until VCC-OVP voltage gate clock is off VCC Zener clamp VCC-CLAMP IDD=10mA Voltage Current Sense Input(Source/Sense Pin) Section Leading-Edge Blanking TLEB Time T_SS Soft start time From Over Current Over Current Detection Occurs till the Gate TD-OC and Control Delay drive output start to turn off Peak Threshold Voltage VTH-OC for Current Limit ZSENSE-IN Input Impedance CV Section System Nominal switch FOSC_Nom frequency COMP=0V,FLTR=5 FOSC_Startup V ??f/Freq Frequency jitter range Error Amplifier Section

    VREF_EA Reference voltage for EA Gdc DC gain of the EA Max. Cable COMP=2V,FLTR=0 I_FLTR_MAX compensation current V Power MOSFET Section MOSFET Drain Source VGS=0V, BVDSS Breakdown Voltage IDS=250uA RDS-ON Static Drain to Source VGS=10V, On Resistance IDS=1A GS288


     8.6 5 2.5

     10 20 3.5

     V uA mA

     27.5 30.5

     29.5 32.5

     31.5 34.5

     V V

     540 10 150

     ns ms ns


     900 50 60 14 +/-4


     mV Kohm KHz KHz %


     2 60 42


     V dB uA

     600 4.5

     650 5.0

     V ?







     High Precision CC/CV Primary-Side PWM Power Switch

     Operation Description

     GS288D is a cost effective PWM power switch optimized for off-line low power AC/DC applications including battery chargers and adaptors. It operates in primary side sensing and regulation, thus opto-coupler and TL431 are not required. Proprietary built-in CV and CC control can achieve high precision CC/CV control meeting most adaptor and charger application requirements. Startup Current and Start up Control Startup current of GS288D is designed to be very low so that VCC could be charged up above UVLO threshold and starts up quickly. A large value startup resistor can therefore be used to minimize the power loss in application. Operating Current The Operating current of GS288D is as low as 2.5mA. Good efficiency is achieved with the low operating current together

    with Multi-mode control features. Soft Start GS288D features an internal soft start to minimize the component electrical over-stress during power on startup. As soon as VCC reaches UVLO (OFF), the control algorithm will ramp peak current voltage threshold gradually from nearly zero to normal setting of 0.90V. Every restart is a soft start. CC/CV Operation GS288D is designed to produce good CC/CV control characteristic as shown in the Fig. 1. In charger applications, a discharged battery charging starts in the CC portion of the curve until it is nearly full charged and smoothly switches to operate in CV portion of the curve. In an AC/DC adapter, the normal operation occurs only on the CV portion of the curve. The CC portion provides output current limiting. In CV operation, the output voltage is regulated through the primary side control. In CC operation mode, GS288D will regulate the output current constant regardless of the output voltage drop. Principle of Operation To support GS288D proprietary CC/CV control, system needs to be designed in DCM mode for flyback system (Refer to Typical Application Diagram). In the DCM flyback converter, the output voltage can be sensed via the auxiliary winding. During MOSFET turn-on time, the load current is supplied from the output filter capacitor CO. The current in the primary winding ramps up. When MOSFET turns off, the primary current transfers to the secondary at the amplitude of

     IS =

     NP ? IP NS

     The auxiliary voltage reflects the output voltage as shown in fig.2 and it is given by

     VAUX =

     N AUX ? (VO + ?V) NS

     Where ?V indicates the drop voltage of the output Diode.







     High Precision CC/CV Primary-Side PWM Power Switch

     Fig.2. Auxiliary voltage waveform Via a resistor divider connected between the auxiliary winding and COMP (pin 3), the auxiliary voltage is sampled at the end of the demagnetization and it is hold until the next sampling. The sampled voltage is compared with Vref (2.0V) and the error is amplified. The error amplifier output FLTR reflects the load condition and controls the PWM switching frequency to regulate the output voltage, thus constant output voltage can be achieved. When sampled voltage is below Vref and the error amplifier output FLTR reaches its maximum, the switching frequency is controlled by the sampled

    voltage thus the output voltage to regulate the output current, thus the constant output current can be achieved. Adjustable CC point and Output Power In GS288D, the CC point and maximum output power can be externally adjusted by external current sense resistor RS at S pin as illustrated in Typical Application Diagram. The output power is adjusted through CC point change. The larger RS, the smaller CC point is, and the smaller output power becomes, and vice versa as shown in Fig.3.

     Fig.3. Adjustable output power by changing RS Operation switching frequency The switching frequency of GS288D is adaptively controlled according to the load conditions and the operation modes. No external frequency setting components are required. The operation switching frequency at maximum output power is set to 60KHz internally. For flyback operating in DCM, The maximum output power is given by

     Po MAX =

     1 2 L P FSW I P 2

     Where LP indicate the inductance of primary winding and IP is the peak current of primary winding. Refer to the equation 3, the change of the primary winding inductance results in the change of the maximum output power and the constant output current in CC mode. To compensate the change from variations of primary winding inductance, the switching frequency is locked by an internal loop such that the switching frequency is

     2009-12 V1.8





     High Precision CC/CV Primary-Side PWM Power Switch

     FSW = 1 2TDemag

     Since TDemag is inversely proportional to the inductance, as a result, the product LP and Fsw is constant, thus the maximum output power and constant current in CC mode will not change as primary winding inductance changes. Up to +/-10% variation of the primary winding inductance can be compensated. Frequency jitter for EMI improvement The frequency jitter (switching frequency modulation) is implemented in GS288D. The oscillation frequency is modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI and therefore eases the system design. Current Sensing and Leading Edge Blanking Cycle-by-Cycle current limiting is offered in GS288D current mode PWM control. The switch current is detected by a sense resistor into the S pin. An internal leading edge blanking circuit chops off the sensed voltage spike at initial internal power MOSFET on state so that the external RC filtering on sense input is no longer needed. The PWM duty

    cycle is determined by the current sense input voltage and the EA output voltage. Gate Drive The internal power MOSFET in GS288D is driven by a dedicated gate driver for power switch control. Too weak the gate drive strength results in higher conduction and switch loss of MOSFET while too strong gate drive compromises EMI. A good tradeoff is achieved through the built-in totem pole gate design with right output strength control. Programmable Cable drop Compensation In GS288D, cable drop compensation is implemented to achieve good load regulation. An offset voltage is generated at COMP by an internal current flowing into the resister divider. The current is inversely proportional to the voltage across pin FLTR, as a result, it is inversely proportional to the output load current, thus the drop due to the cable loss can be compensated. As the load current decreases from full-load to no-load, the offset voltage at COMP will increase. It can also be programmed by adjusting the resistance of the divider to compensate the drop for various cable lines used. Protection Controls Good power supply system reliability is achieved with its rich protection features including Cycle-by-Cycle current limiting (OCP), Over load Protection (OLP) and over voltage clamp, Under Voltage Lockout on VCC (UVLO). The OCP is line voltage compensated to achieve constant output power limit over the universal input voltage range. At overload condition when COMP input voltage exceeds power limit threshold value for more than VCOMP-OL, control circuit reacts to shut down the switcher .Switcher restarts when VCC voltage drops below UVLO limit. VCC is supplied by transformer auxiliary winding output. When VCC ramp up to OVP threshold voltage (VCC-OVP =29.5V), the output of GS288D will be shut down, when VCC drops below UVLO limit and Switcher enters power on start-up sequence thereafter. When VCC higher than VCC clamp threshold voltage(VCC-CLAMP), the internal VCC clamp circuitry(10mA) will clamp VCC to 31.5V, and the output of GS288D is shut down also.







     High Precision CC/CV Primary-Side PWM Power Switch

     Characterization Plots

     The characteristic graphs are normalized atTA=25?æ.

     VCC-ON (V) vs. Temperature(C)

     15.8 15.3

     14.8 14.3 13.8 -40 -10 20 50 80 110





     I_FLTR_COMP (uA)

     FOSC (KHz)




     High Precision CC/CV Primary-Side PWM Power Switch

     Package Information



     A A1 A2 b b1 D E E1 e L eB ?È?

     Millimeters Min.

     0.381 3.175 3.302 1.524 0.457 9.017 9.271 7.620 6.223 6.350 2.540

    2.921 8.509 0? 3.302 9.017 7? 3.810 9.525 15? 0.115 0.335 0? 6.477 0.245

    10.160 0.355 3.429

     Inches Max.

     5.334 0.015 0.125 0.130 0.060 0.018 0.365 0.300 0.250 0.100 0.130

    0.355 7? 0.150 0.375 15? 0.255 0.400 0.135











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